Bump-fire devices and techniques have been around a long time.
Also, cranks that fit against the trigger were popular for a while.
This is just another one of those type devices - but it looks like a better made product. This just means that the ATF will change the definition of "machine gun" like they did in 1983.
New rifle mimics machine gun's rapid fire -- and it's legal - Sep. 12, 2013
This one was legal, and then later determined to be illegal:
