Chi Sau (Sticky Hands) is a two person drill used to practice fluid control of motion between bong sau, tan sau and fok sau, while "sticking" to an opponent. The real skill to this exercise is learning the timing of another while controlling your own footwork, sensitivity, and technique. If you learn control of yourself, then you can control an opponent through their own technique. Chi Sau is considered the backbone of Wing Chun and is also referred to as "sharpening the bit". It should not be confused with sparring (gor sau).
Lop Sau (Grabbing Hand) is a partner drill in which one person (the attacker) simultaneously executes a controlling motion (initially lop sau) and an attack (initially a punch). What is important in this exercise is to execute each technique so that both hands work together (the control and the punch) and to time the motions so that both hands complete there movements at the same time. Lop Sau teaches the practitioner to use one hand to control or trap as the other hand attacks. While the attacker learns to attack, the defender in Lop Sau learns to block with the correct position and the best blocking line.
Asking hands or (Man Sau) is a Wing Chun exercise that develops skill in closing the gap (meaning the distance between you and your opponent) using the step slide stance learned in the Chum Kiu form, trapping the opponents hands, timing techniques, blocking, and taking and regaining the inside line. There are many ways of trapping your opponent hands. One way is by knocking his hands off the center line. Another way is by pulling his hands off the center line, and finally a third way is by trapping both of his hand on the center line as shown in these pictures.