It's sleight of sound. It's a nifty little device that allows you to pull off a different kind of ruse. Combined with pantomime, the auditory illusion it creates allows you do effects you couldn't do otherwise.

You also get to add extra phases to existing routines with soda bottles or beer bottles.

Btw, I find this device easier to have in finger palm than classic palm.
And get an elastic pull separately if you want a way to ditch the device; it is not included.

Be careful to not block the speaker, otherwise the sound may end up muted.