#1
When slappinig do i use my thumb to hit the string against the fret board or do i go up and down like using a pick. I use my thumb to hit the string onto the fret board but after waching a bit of a tutorial vid it showd him slapping up and down, is this correct or are both ways correct and its down to preferance?

thanks
#2
The method of slapping is just like you said- hitting the string with your thumb against the fret board.
picking up and down with your thumb is a different technique called double thumping. victor wooten uses it a lot.