In wing chun we strike the center line, so when an opponent moves his hand from his centerline there is an opening to strike. If you don't strike that opening and waste time trying to follow the persons hand, you waste time blocking when you can strike you are chasing. Basically, it is following your opponents hand.
Now, this is dangerous if you fight someone who fakes a lot and has very fast hands.
Another concept that we would utilize also was when the reference points are not being referenced correctly we would move to a San Da timeframe and then regain the center.
So we had 4 different time frames that were utilized during Chi Sau which was:
Kiu Sau
Chi Sau
Chi Kiu
San Da
These were more conditioned responses based on energetics and structures as opposed to drilling formats. (although in the beginning we started from a drill format to gain an understanding)