Hit the thumb against the string close to the fingerboard rapidly and firmly, twisting the wrist then releasing the thumb immediately after contact with the string. This rhythmic attack is characteristic to slap playing and is one of the selection of sounds that create the complete slap .
A long sustained note plays an important role in the outcome of the sound. It is achieved by the accuracy and firmness of the movement not by the force used. Smashing the string will not produce a clear sound and will in fact create unwanted background noise. Control of noise is imperative.