The rule revolves around this statement
Specifically, these devices convert an otherwise semiautomatic firearm into a machinegun by functioning as a self-acting or self-regulating mechanism that harnesses the recoil energy of the semiautomatic firearm in a manner that allows the trigger to reset and continue firing without additional physical manipulation of the trigger by the shooter.If only it were true. As the gun rocks back and forth, the shooter must be present and engaging the trigger on each shot. Absent the shooters finger, the thing doesn't work.
H/T to Market Ticker for the link.