As jlq stated above, the "5 thunder punches" movement is a five-punch combination that LT added to his Biu Tze form back in the 80s. It consists of a straight punch, a frontal double punch, and finally, a very heavy turning double punch delivered with the hands held and moved as though swinging an axe.
It feels great to do, ending with a very strong double hit using a lot of body torque. It also looks great in demos, but in actual application I find it impractical since nobody stands still long enough to let you deliver the whole sequence (this can be seen in the clip below as well). There are occasionally times when you can set up and deliver the torquing double-punch though, and like the turning high low "scissors" gaun sau, that precedes it in the Biu Tze form, it is a good way to practice putting body torque into your technique.
Here's a clip of some guys from a WT spin-off group using the "5 thunder punches" in a choreographed "chi-sau section" (4:20-5:00, and again at 5:55-6:40):
