When using the peacemaker gun (purple ammo which fire lightning balls) hold R1 to charge gun but not fire. While running, press dash (square) and the gun will whip behind your back, over your shoulder, and down at the ground as you rush foward. The trick itself takes good timing and a bit of practice. If you release R1 at exactly the right time (right after pressing square)the gun will fire straight back. This is good for killing pursuing crimson guards. Other good times to fire include when the gun is halfway between backwards and straight up (hits pursuing cars) or pointing into the ground (hits surrounding enemies). This trick is more effective (and fun) when you have infinite ammo.