Position One:
The Red squad is your target. Your Blue unit has moved up, but they can't reach the Red unit for an assault. The Yellow unit is an IC who hasn't moved yet.
The Illegal Move:
The IC charges the Red unit during the assault.
The Red unit follows up.
Then the Blue unit, because of the Red unit pile in, is now within assault range.
This is illegal becase all assaults must be declared and the assaulting player's units moved before the other guy has to pile into the assault. This is generally referred to as the "Assault Pull" because you have 'pulled' the enemy into range of a squad that would not otherwise be able to assault.
The Legal Move:
Start.
This time, the IC first moves within 2" of the Blue unit.
The IC automatically joins the unit at this point, per the movement rules, and is within assault range for the assault phase - meaning the squad he just joined is also in range.
Some things to keep in mind here... An IC with a jump pack or some other means of moving fast is great for pulling the legal move out of your bag of tricks mid-game. If he follows behind several units, he can spring forward to close the assault gap at any given time during the game when your opponent thinks he's still safe. But remember to keep him at least 2" away from the squad you intend to join in this fashion before you attempt this move or else he'll only be able to move as quickly as the slowest member of the same unit - completely wasting his speed.
Also, keep an eye out to make sure you don't fall prey to the illegal assault pull. For convenience sake, it is pretty common to go tit-for-tat during your opponent's assault phase. But don't let him pile any new units into that assault once you've piled in. Ask point blank whether or not he intends to charge anyone else into that same combat before you move your models in the reaction move.