hey Darmin
I got the following from
Wheat's Bass Book:
The SlapTo slap a string is to strike it with your right hand thumb, so that it makes a distinctive, percussive sound. Doing this effectively requires that your thumb be somewhat perpendicular to the strings. Many players prefer to slap with their thumb pointed up [towards the face], some point their thumb toward the floor [e.g. Flea], some do both, depending upon the line.
The best place to slap the strings is right where the neck of the bass joins the body. In order to hold your bass comfortably so that you can strike this area with your thumb perpendicular to the strings, you'll probably have to strap your bass a little higher than you would to play fingerstyle (if you're slinging it around your knees like the guy in Spacehog, you're going to have to modify your appraoch). Players who play both fingerstyle and slap/pop (which is most of us) have to find a way of strapping the bass which allows for both of these approaches.
Start by resting your right hand thumb lightly on the E string. Your other fingers can either by spread out or curled under. Go ahead and let them rest on the other strings (they'll help to dampen them). Most of the motion in a slap should come from your wrist (not your forearm), so when you lift your thumb, you'll let your wrist rotate naturally (so that your pinky is still resting on or near the string, even though your thumb lifts off of them). Now, practice lifting your thumb off of the string and bringing it down quicky onto the E string, so that it makes a percusive sound. You don't even need a bass to do this. You can hold your hand across your chest to practice it, or you can practice it on a nearby chair arm or table top. This is the basic motion. Remember to make sure that your thumb bounces off of the string after you hit it (so that it can ring). If your strings are reasonably new, they'll have a natural bounce to them (like a drum head). The deader they are, the less bounce they have and the more effort it takes to get a good slap sound.
