The first situation is Tan Sau vs Punch. Imagine that the attacker punches at you. You use Tan Sau and meet the attack by quickly pushing forwards and making contact at your opponents wrist. This way you cause your opponents fist to slide off target while making him fall forwards. The result is that without pulling your hand back. Your Tan Sau becomes a punch, striking your opponent on the jaw.

The next one that we want to talk about is Fuk Sau. Your Fuk Sau rests on your opponents arm. In this situation your foearm and wrist are relaxed but you have to use enough energy to support the Fuk Sau so that it just touches your opponents arm. You just should not push down or to the side, or you opponent will fel his strength use it against you. This movement is mostly used for sensing your opponents energy, so that when he or she moves to attack you feel it. This movement can also be used when you want to protect your centreline if your opponent is pushing forward.

Source: http://gowingchun.com/self-defense-in-siu-lim-tao.html