some more rules:
Players are not allowed to shuffle their cards. That means if someone slaps a pair of cards, those two cards will remain next to each other. So if you remember the card before them, you will know if that pair is going to come up again.
However, if you know that you yourself are going to play a slappable card, you must play the card, then touch your deck with your slapping hand before you are allowed to slap the pile. This prevents you from just slamming your card down on the pile and picking it up.
If you slap the pile when there is not a slappable combination of top of it, you must put the top two cards of your deck on the bottom of the pile. If you have no cards, you simply get a dirty look and will be disqualified from the game if you do it too often.
With two players, you can play that any two cards whose sum or difference is seven is slappable. (eg 3&4, 9&2, 6&A) This makes it more a game of reflexes and allows the better player to end the game before it becomes tiresome.