There are many approaches to slap bass. Some slap bass players like the thumb to be parrallel to the strings, while others like the thumb to be perpendicular. Some slap bass players like to strike the thumb through the string, while others like to bounce the thumb off the string. Some slap bass players like to keep their hands in a close fist with their thumb sticking out, while others like to keep their hand in the shape of a claw.
The way your thumb strikes the bass string does affect your slap tone, swing, stamina, and speed. However, there is one aspect of slap bass that is often overlooked.
How To Relax Your Wrist
Keeping your wrist relaxed is one of the most overlooked aspects of learning how to slap the bass.
A relaxed wrist allows you to last longer, more easily change the direction of your thumb, and allows your string to vibrate when the thumb strikes it.
You will also find that by keeping your wrist relaxed, less power is needed to get a sound.
Start out by taking a towel, twisting it up, and flicking the towel.
In order for a towel to flick at top speed with full force, your wrist has to be completely submissive to the motion, in order for the weight of the hand to flick.
Take note of how your wrist feels. It should feel effortless. The real effort is coming from the bigger parts of the arm and the momentum that is created when the weight of the hand is traveling.
Slap bass is very similar, only that the motion of the hand is going inward instead of outward.
I am not suggesting that you flick your wrists in order slap bass. What I am suggesting is that you direct more attention on how to allow your wrist to stay out of the way.
A great way to see this concept in action is to see a someone playing a Celtic Frame Drum. Their wrist is free and relaxed.