To "lap-slap" a rhythm you take the rhythm of the left hand and the rhythm of the right hand and you slap them out on your thighs VERY SLOWY while saying something like: right - left - together - left etc. Then you gradually work up to full speed.
This year my student Joe's recital piece has four different sections with very complex rhythms. He tried the "lap-slap" technic that he learned several years ago and again it helped him a lot.
Here is an audio of Joe first demonstrating the four different rhythm sections and then playing the entire piece.