PC clamping is basically when you go to pee if you want to stop the flow you squeeze the pc muscle making the flow stop and then relax and the flow flows. PC xclamping really is any form of flexing this pc muscle . Some people do sets of 100 quick clamps others do sets of 10 holding the squeeze for 10 seconds etc etc.
Its not an exact science and is really down to what you prefer. This technique can also help you to last longer as when you feel the urge to come you squeeze the ass of your todger muscles and should be able to divert the point of no return, for this it definately has advantages.
Give it a go, you need to persist though. A common analogy is the gym one and that is you go to the gym for a week you wont notice much change. Do it consistently for 3 mths and thats a different story.
JT