In music, the term slap tonguing refers to a musician playing a single-reed instrument such as a clarinet or a saxophone employing a technique to produce a popping sound along with the note.
Get rid of as much air in the oral cavity as you can and seal off the lip so that you have an airtight fit.
It needs to pop downward away from the roof of the mouth to get the most volume, do not blow air through the horn and do not inhale when you release your tongue.
The first recorded appearance was by Stump Evans, the C melody saxophone player in the King Oliver band.
Other famous players who used the technique were Rudy Wiedoeft, Coleman Hawkins and Fess Williams.