'Controling' npc troopers

I know this works on the PS2, and I haven't played the Xbox version in a while, but they were virtually the same. Controlling NPC's mean that you can give them orders, by pointing your target reticle on one of your own troops and hitting your controls on the left side of your controller in one direction. That direction corresponds with an order.

For example, 'Up' will give the order for a trooper to follow you. That is, until he comes within range of an enemy, then he will try to wipe them out and rejoin you later if you both survive and no more men remain around you.