From The Federalist:

As Davis noted, in 2010 the agency said bump stocks weren’t “machineguns,” that a bump stock “performs no automatic function when installed. In order to use the installed device [the bump stock], the shooter must apply constant forward pressure with the non-shooting hand and constant rearward pressure with the shooting hand.”

Rejecting its 2010 determination, the BATFE now says that a bump stock causes a semi-automatic firearm to fire “in a manner that allows the trigger to reset and continue firing without additional physical manipulation of the trigger by the shooter.”