Contact mind reading -- sometimes referred to as "full-contact mind reading" -- is a form of mentalism in which the magician tackles a spectator so hard that the spectator's thoughts fly right out of their head and into the magician's.
There are a number of ways to make this effect work:
1) If the spectator has picked a card, the tackle is just an excuse to get a look at their selection. You usually have plenty of time to take a peek while they are flailing to maintain balance, falling over, or struggling to get up.
2) If the spectator has drawn a picture and sealed it in an envelope, casually open the envelope and look at the drawing during the tackle. The larger motion will hide the smaller motion.
3) If you want to mentally "see" the serial number of a spectator's bill, do a little "pre-show" work. In this case, ask your intended target's mother or spouse whether the target is carrying a bill with a serial number they happen to know (they will, more often than not -- mothers in particular know all sorts of things you wouldn't expect). During your act, tackle the target and then reveal the serial number (secretly reading it off your shirtsleeve or the back of your hand).
4) If you tackle someone hard enough, sometimes they will just tell you stuff to get you off of them.