At the end of each close combat, models in units that are locked but which are not engaged in combat themselves MUST move up to 6" in an attempt to contact enemy which their own unit is locked with. If they cannot engage the enemy, they must get as close as they can to them without moving out of unit coherency. Neither the presence of enemy models, nor terrain, affects reinforcement moves. Unlike in a charge, reinforcing models may move through obstructing terrain and models in order to engage. It can be imagined that the fighters at the front of the combat are becoming increasingly embroiled and making such movement possible. Note that the other restrictions for making a charge still apply, so models must contact enemy models which are not already engaged in combat in preference to those which already are and must end their moves in coherency with another member of their own unit.
Diagram 9. Pile In: Model 1 must move to engage the unengaged enemy model. Model 2 can pile in to help a friendly model as no enemy models are left unengaged.
Whenever both sides in an engagement have extra models to move, the player whose turn it is moves first.
Important note: Once a unit is locked in combat it may only make Pile In moves and may not move in the Movement phase. If for some reason (mass carnage usually!) Pile In moves will not enable the unit to engage or support against any enemy models, and the unit has no models engaged in combat itself, it is no longer considered to be locked in combat. In this case it makes a consolidate move instead - the close combat comes to an end as the unit finds itself with no enemies to fight.
Designer's note: This last bit is just to cover some situations that come up (usually in multiple combats) where a unit slaughters everything and the Pile In move won't get it into combat with the remnants of the enemy (as they'll often be surrounded and unable to pile in themselves). In this case it keeps things cleaner to simply consolidate with the unit instead.