Slap bass is funk's signature sound. The thumb hits the lowest strings hard against the fretboard, producing a percussive "thump." Larry Graham (Sly and the Family Stone) invented the technique in 1969 because his band's drummer quit and he needed to add percussion.
The thumb motion
Hold your hand in a "thumbs up" position. Rotate your wrist so the side of your thumb (the bony part on the outside of the joint) faces the strings. Strike the string by rotating the wrist, not by flexing the thumb. The thumb bounces off the string after striking.
The strike is closer to the neck than fingerstyle plucking, around the 24th fret area or above the neck pickup. The thumb hits the string and immediately rebounds, leaving the string free to vibrate.
Drill the thumb slap
Open E string. Slap it with your thumb. Listen for the "thump" sound: percussive, slightly muted, but with the open E note still audible.
70 BPM. Slap the open E string on each beat. Loop for two minutes. Listen for consistency: each slap the same volume, the same tone.
The two main thumb targets
In a slap line, the thumb usually slaps the E or A string (the low strings). The "pop" (next lesson) handles the G or D string (the high strings).
Wrist, not arm
Beginners try to slap by moving their whole arm. Wrong. The motion is a small wrist rotation, less than 90 degrees. The arm stays still; the wrist flicks.
If your forearm tires after one minute of slapping, your technique is wrong. Slow down and isolate the wrist motion.
(Slap lines almost always use the minor pentatonic, useful reference.)
Next: the pop. Index or middle finger pulls the high string and lets it snap back.