First off, let's set the scene. Below you can see an enemy unit hunkered down in cover (the boxes in red) while taking up firing positions around an objective (the yellow circle). This can be a tough unit to dislodge because of the benefits of cover. Throw in additional special rules such as Feel No Pain (plague marines), Stealth (space marine scouts), or Counter Charge (grey hunters), and even a small unit stands a good chance of keeping your fast objective grabbing squads from pushing the enemy off the objective.
Now let's look at some options...
(1) Assault Unit: This kind of unit is, not surprisingly, dedicated to getting into hand-to-hand and duking it out with the enemy. The benefit to dropping in an assault unit in this situation is that the enemy guarding the objective most likely will not be a combat unit. Generally the kinds of units left on objectives are geared towards surviving lots of enemy fire by hiding in cover and/or having lots of armor. They also generally have at least medium ranged weapons (like bolters) if not heavy weapons (missile launchers, mortars, etc.) so that they can continue to contribute to the battle while babysitting the objective. And, since it is also rare for units geared towards shooting to be good at close combat (although exceptions to exist), you're assault unit should have a decent chance of pushing the other guy off the table... provided you can survive splash down and the initial charge.
(2) Big Nasty Unit: This kind of unit is something that will make the enemy shake in his boots, regardless of what he has sitting on the objective. Good examples of this are dreadnoughts in a drop pod, deep striking thunder hammer terminators, deep striking demon princes, and tunneling trygons. The reason these types of units are great for vertical envelopment is that they can show up, take a lot of brutal firepower from the enemy the turn they show up, and then survive long enough to do a couple rounds of slobber-knocking close combat to move the enemy off the objective.
(3) Fire Support Squad: This is a firehose unit. The point of this kind of squad is to drop in from above, and then hose the enemy with overwhelming firepower rather than launching an assault. The good thing about this kind of unit is that unlike the other types, the firehose squad doesn't need a turn to start being effective at its role - they show up and start firing right away. Sternguard and Outflanking firedragons are good examples of this kind of unit as both the ability to use massed 'special' firepower that can cut through heavily armored foes protecting objectives.
(4) Glass Hammer: Glass Hammer units are generally fast and ferocious units that can cause lots of damage quickly, but do not have much staying power. Landspeeders and first edition Dark Eldar ravagers are good examples of these kind of swift and fragile units that can swoop in at a moments notice (both of these exemplars have the deep strike ability) and then unload nasty firepower to take down the enemy quickly.
Don't Leave Your Forces Exposed
The key to using any of these units, however, is that you have to use them in concert with one another. If you drop a single assault unit, or even a single big nasty unit, into the backfield of your enemy, do not expect that unit to last very long. Vertical envelopment is a scary prospect. Nobody likes having their lines penetrated by enemy units -- people prefer to think in terms of controlled battle lines where you can try to keep the enemy in front of you while you try to out-maneuver him. Essentially, even 40k players prefer to fight in a battle where the other guy moves in a near warhammer fantasy battle approach to the game; it's easier to fight that way because you can see where everyone is, and you can easily predict where and how the other guy's forces are going to move. Vertical envelopment is dangerous because you have to keep your lines tighter in order to provide protection to your squads. Vertical envelopment means that your backfield support units can get cut off, and your own battle lines can quickly get caught up in a cross-fire situation where they can only take cover from half of your army's shooting -- which is not a good position to be in.
Accordingly, when you threaten the enemy by dropping troops into his backfield, he will generally respond with overwhelming firepower to eliminate (or severely cripple) your unit before it can become a serious thorn in his side. If you don't want that unit to get creamed the minute he enters the battlefield, then you need to use your outflanking/deep striking squads in mutually supportive groups.
Mutual Support
What I mean by mutual support is that you need to create situations on the battlefield where the other guy is left with Morton's fork -- that is, you have to create situations where he is left with equally unpleasant alternatives. Look at the situation above. If you drop in a single assault squad, the enemy's choice is pretty simple: mass firepower on the unit until it is effectively broken or destroyed. And the same goes with any of those units. If you drop any single unit into the enemy's crosshairs, you're leaving him a pretty simple choice: ignore your unit and die for his Emperor or make you die for yours.
Conversely, if you drop two assault squads down, then the enemy has got twice as many problems to deal with -- but in the end, he really has the same choice: gun down one assault squad and then move on to the other.
The real trick to creating Morton's fork is to give the enemy an actual choice -- make him choose which club you're going to clobber him with. If you drop in an assault squad and a big nasty unit, then the enemy has to decide which of your units is the bigger threat. And frankly, it doesn't matter to you, because his concentrating on either threat means that you're still going to hit him in the face with the other unit. But because he is choosing which unit you are going to hit him in the face with, you will cause him actual concern and he's going to try to do all kinds of math in his head trying to make the "right" decision. Any time you make your enemy do math in his head like that, he's back on his heels, and he's going to start making mistakes.
Good Pairings
I like to use certain deep striking pairs in concert with one another:
Glass Hammer + Assault Unit/Big Nasty: I find a great combo to be a fairly fragile unit, which can be deep struck or just sent racing across the table, coupled with an assault unit or big nasty unit dropped in right next to the enemy. This is a good pairing for a couple of reasons. First, timing an effective reserve roll is near impossible until late in the game. By using a speedy glass hammer, like a landspeeder, when the big nasty unit does drop in, then you use that same movement phase to just race that glass hammer right down the throat of the enemy. The bad guy then has to decide if he is going to deal with the scary unit that just dropped in, and is certainly going to start messing him up on my next turn, or try to deal with the glass hammer that may not quite be in range yet, but will certainly cause problems for me if I don't deal with him now (such as a round of shooting before the scary unit pounces on me in an assault). It's a tough choice, and it doesn't have a great answer, which is a good position for you to be in.
Assault Unit + Big Nasty: While these two units have effectively the same purpose, this kind of coupling causes math headaches for the other guy. Is it more likely that he can gun down your genestealers that just outflanked him and are closing in on him, or his he actually going to be able to put 6 wounds on that trygon that just popped up? It's a tough choice, and again causes the enemy to second guess himself. If you can manage to get these units to pop up on either side of the enemy, all the better, because then he can't just run away without running towards the battle lines, and thus towards the bulk of your army.
Support Unit + Big Nasty/Assault Unit: If you've done it correctly, then the support unit will pop in and make its presence known the moment it appears. While the support unit is not an immediate threat for taking the objective away from the enemy, it will soften up the defender -- every good 40k player knows that volume of fire is a very dangerous thing. Further, if he decides to ignore the support unit in favor of the assault unit or big nasty, and he doesn't successfully take down the other threat, that support unit is going to get a second chance to soften up the defenders before the big assault takes place, which isn't great. Further, that support unit will also continue to be a thorn in his side by harassing anyone and anything within firing range - which is generally much more substantial than the assault range on the other unit. Support units not only threaten his backfield, but will also create pinch points in his battle lines by creating cross fire threats against his main force, and threatening to create scenarios where falling back units become trapped before they can rally, or just ushered off the board.
One pairing I don't like to use is a support unit with a glass hammer. The reason I don't like it is that a support unit is generally best when dropped in the middle of the combat zone between the two forces rather than dropped on a flank -- if you put them on a flank then they can't create cross fire scenarios or trap falling back enemies as easily. Since that is also the same direction your glass hammer is generally coming from, then the enemy really isn't facing a vertical envelopment situation -- it's just a fast paced frontal assault.
Conclusion
Vertical envelopment is scary because it creates chaos in the enemy lines. Troops that 'shouldn't be there' are suddenly crawling through the enemy fire lanes and threaten to swallow whole sections of his army whole. But if you want to do it succesfully, you have to lend your units support. Lone squads don't generally fare well because they can be easily overwhelmed. Further, multiples of the same kind of squad are better, but they fail to take advantage of the real chaos you can cause by making the enemy second guess himself by offering him two or more horrible choices. The best way to use vertical envelopment is by dropping in multiple threats and trapping him between the proverbial rock and hard place. Pull him in multiple directions by creating threats on more than one front, and make the enemy decide what you're going to hurt him with by offering up different types of nasty units.
