Well, during my playing days (I retired in 2005) it was any part of the hand, so long as there was a distinct connection with the ball as opposed to a kind of push or throw. Using the heel of the hand was most common. Rules change quite often though and I don't follow too closely these days. The sport has gone to shit, in my opinion. Most games are turgid affairs. Slow moving and defensive, the opposite of what it should be. Hurling, however, is still a joy to behold.
What is the web telling you?