If you study Wing Chun, you must have heard about Pak Sau. After learning the basic techniques: Tan Sau, Bong Sau & Fuk Sau, you will learn some more self defence methods where you can apply the principles to fighting. Remember, the Wing Chun principle is to use as little energy as you can; moreover, you should use your enemies energy! Pak Sau follows this principle and results in powerful attacks. (QI Magazine)
When the enemy attacks you with a punch. His arm will be tense. As he comes toward you, just receive his energy with Tan Sau.
Then bring your Wu Sau (protective hand) straight forward, slapping his forearm just below the elbow. The enemy will automatically jerk forward losing his balance.
Then, your Tan Sau punches straight forward, so that the enemy will suffer all the energy from you and from his own punch!
This is the principle of the Pak Sau technique. Practise regularly and you will find a lot of benefit for self defence and sticking hands.