Take a look at the Martial Arts Talk thread and a topic "Advanced punching" started by mike88nbd.
The general rule on training is NOT to damage any of the nerves, in the hands especially. Hence the use of chi gung and medicinal ointment ("dit da jow" --- cantonese pronunciation) It is an ancient warning that damaging the nerves in the hands will affect your eyes in a negative way. Thrusting your fists / fingers into rice / beans / iron beads etc with no proper guidance is extremely dangerous, because what you see in books, videos is just the act, but maybe not the precaution these people take to protect their hands.
There were guite a few good basic methods posted by members before the recent overhaul of the system, after which some of these were "deleted" during the process. A pity.