This technique is usually employed by bass guitarists but can be adapted to the Jazz fingerpicking guitar style. Thumb slapping requires the side of the right hand thumb to strike the string against the fretboard as a note is fretted with the left hand.
The thumb strikes the fretboard at the sound hole end, then immediately pulls away from the string.
Listen to the recording of the next example to hear the effect of the slapping technique. A slap is indicated with a down arrow.