The guts are rather simple, after pressurizing with propane, the piston locks against a stack of metal "beads" which are assembled onto the pin. When the pin is pulled, the stack remains in place due to force from the piston, but, with enough inertia from impact, the stack collapses allowing the piston to move upward and release gas through the grenade's "barrel" which is essentially a spiral channel containing 200 bb's.
It's a lot like a coax barrel sealer with a mechanical sear rather than a pilot valve.
I have no use at all for a thing like this but I figured for $30 I might as well get it... and it is thoroughly fun to use. The videos you see online really don't do it justice; the "blast radius" and velocity behind the bb's are impressive. There's also a timed version which I have not used.
