Near the end of Chapter 2, you will come across a jump pack. When you put this on holding will cause you to launch into the air. If you look down you will see a group of arrows pointing to a spot beneath you. If you press when you see these arrows you well jet towards the ground and smash into it, performing a Ground Pound.
Most regular boyz should die in one Ground Pound, but the Tankbustas (the guys with rokkits) will only be stunned by a ground pound. You must damage them before you attempt to smash them, or else you can simple Ground Pound them twice to kill them.