In the heimlich maneuver the fist is pressed into the victim's abdomen with what?

The Heimlich maneuver (now called abdominal thrusts by the American Red Cross) is done by making a fist (pac-man / thumb facing the stomach) with one hand and on the stomach and the other hand, open-faced, helping to push the fist up into the diaphragm.

That in-and-up ("J") motion applies pressure to the diaphragm, essentially knocking the wind out of you (your goal, of course). That motion should be done as hard as possible. To give you an idea of the amount of force needed, a colleague of mine did abdominal thrusts on his brother's son, an 8 year old boy who had choked on a piece of hot dog. He did four abdominal thrusts with no success. On the fifth try, the thought that he said went through his mind was "Oh my God - if I do this any harder, I'm going to break this kid's spine" and sure enough, the piece of hot dog came flying out. It takes a lot of force!