To be honest, I rarely do these. I tried to get my 6 year old to do the video but she is in a phase of saying 'no' so I was out of luck. They are fun. They require control and well crab, done well, shows the strength of your neck (and we all need stronger necks, though this is not the exercise to make your neck stronger).
Primarly focus: Lengthening the back body
Secondary focus: Control