Kao Sao (rotating arm) is like a Bong Sao that continues to rotate round a bit more (often whilst the hand locks on to the opponents wrist).
In Chi Sao you can use it to trap and then turn it into an elbow strike.
In applications, imagine you were grabbed by the throat by the two hands of an assailant. Put a Tan Sao into the middle of the two arms and rotate to Bong Sao and a bit beyond. Then hit'em in the face with your other hand!
The English struggles in translation I think, in the vernacular it's about circling, controlling, detaining. It looks like a bong but used differently to 'open' instead of using elbow high, the elbow's low and it makes a 'screw action'. This opens guards or can be used to 'fold'. Its used a lot in dummy form and all quite subtly the rolling and folding actions employ it also can be used in dummy form accompanying jum to close down arms, it's quite a subtle shape, but is used often!
Whereas bong sau is applied to the outside gate of your opponents arm, kau sau can be used to transition from fook sau (outside gate) - kau sau (looks like bong sau, but wrist is on opponents inside gate) to tan sau/strike on the inside gate. I generally use it to change gates. Ideally, you should not apply lateral pressure to clear the opponents arm, rather you should rotate the arm and the stance to allow the technique to work around the wrist.
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