It's also worth pointing out that the YCKYM , apart from being used to guard the groin and launch rapid kicks along the centreline also has another wonderful use , that of leg locking .
For the benefit of the people that aren't familiar with the leg locking application of the stance , it is basically interlocking your leg with their leg at the shin and sinking your weight down on that leg , it can be used to break their leg at the knee , or if done with less vigour just to force them down to the ground .
Chi gerk should be practiced just as much if not more than chi sau.
An term we use for "Leg locking" is leg trap.
As pointed out by Tenzen, with chi gerk comes leg traps ... just as with chi sao comes hand/arm traps, you can also use your body to effect traps ... but that will be another whole new thread.