On Shangri-La, go to the part where you are under a water fall with a big rock in the middle.

Go to the part of the rock that is facing the edge of the other water fall.

There should be a brick sticking out of the rock. If you get close to it, it will say DO NOT HOLD X.

If you hold X, a whole bunch of water will flow in and knock the zombies off their feet.

This should be used when there is a whole bunch of zombies near you.