David's Tip of the Day: Different Types of Tremolo and Vibrato - Part 2, Hand Tremolo

Cup your harmonica so that it's not squeezed airtight, but closed nonetheless. Most players hold the harmonica with their left hand and open/close their cupped hands using the wrist of their right hand at a speed slightly faster than a triplet (this depends on the tempo of a song, slower relative to fast tempos and faster relative to slow tempos). When your hands are closed, both the volume is soft and the tone is muted and bassy. When your hand open, the volume is loud and mid and high frequencies are back with a less tubby tone. Both volume and tone are changed in a hand tremolo.

P.S., When the hand tremolo is used, the throat tremolo is generally not (thought it's an interesting effect to use both, but not common).