The usage of pock sau and yeun sau in those drills are designed to develop your palming-off skills, and also (See-lik)test/try power of the opponent's strikes.
Overall these drills develop your body movements, entry and exiting footwork, timing, conditioning, reflexes and the most important bik kiu (force bridge skill) onto the opponent's bridge.
Once you've mastered these drills, it's very hard for anyone to strike at your body or your bridge for that matter. In fact, once you've palm off their destructive strikes with yin powder, your muscle memory will kick-in immedately to deliver deadly pak, pai, pok, gow chui in flowing sequences.
Frankly speaking, it's too bloody dangerous to teach these drills to anyone that has an evil mind, that's why I still kept them close to my chest.