Slaptongue & Smacktongue

Smacktongue

This is often called slaptongue, but I believe this is a more accurate description for a slightly different (more percussive) version that has more of an indefinite pitch like a smack or clap. This effect works best on a low Bb. You can hear King Curtis doing it on King Curtis & Champion Jack Dupree - Blues at Montreux

Place the surface of your tongue flat against the reed and finger a low note. It should feel like if you try to move it away there is a vacuum between your tongue and the roof of your mouth. Now quickly pull the tongue off the reed while and go "ha!".

Slaptongue

This is very similar, and is a more classical technique, common for musical passages in the early part of the twentieth century, especially in some novelty, danceband and vaudeville styles. With slaptongue, more of the actual note is sounded and the tongue is not used so explosively. (Examples and more on this to come).