Within a taal, some vibhags begin with a clap and one (or more) begin with a khali — a wave of the open hand, palm up, instead of a clap. The khali is the "empty" or unstressed section.
This contrast between clap and wave is essential navigation. In Teen Taal the khali falls on beat 9, exactly halfway; feeling it pass tells the performer the sam is now eight beats away. On the tabla, khali bols use the higher, open sounds rather than the resonant bass.
The interplay of tali (clap) and khali (wave) gives each taal its characteristic light and shade.