If you are unaware, bump firing is the process of using the recoil of the weapon to basically make it full auto. I've done it the past with my sks by hooking my thumb through the trigger guard and then into the belt loop on my right hip. A little forward pressure at the front of the rifle and I was wasting lots of money on ammo putting that 7.62 down range!
The good people at Slide Fire Solutions have created an ingenious product that replicates the same bump fire style in a more controlled and safe manner. In fact it is so easy to use some dude had his wife bump fire and put it on youtube for all the trolls to comment on! Here is a little info from their website:
Applying principles that have been used for over 40 years to bump fire, the SSAR-15's radical design enables the shooter to fire an AR-15 as quickly as desired without compromising their own safety or the safety of others around them. Unlike traditional bump firing, the SSAR-15 allows the shooter to properly hold their firearm and maintain complete control at all times. In addition, as a result of the forward movement required to discharge each round, the shooter naturally corrects their point-of-aim for each shot and prevents recoil from pushing the firearm's muzzle upward in an unsafe direction.
In principal the SSAR-15 functions similar to the Gatlin system. When operating a Gatlin system; the shooter operates a crank in a circular motion to activate the firing mechanism. Every 1/5 rotation of the crank will discharge a round. The shooter will then continue forward another 1/5 of a rotation to fire the next round, and so on. With the SSAR-15 the shooter activates the firing mechanism by moving the upper assembly forward to discharge a round. To discharge the next round the operator must again move the upper assembly forward. As with the Gatlin system, an AR-15 equipped with the SSAR-15 will never fire automatically.
I really would love to try one of these products, but at $370 for the AR version I'm a little skeptical. Slide Fire recently released an AK version for about $300, and the vid below really does a great job reviewing the stock. He also did some testing in full auto mode, and it seems like the stock is relatively accurate at zombie spraying distance.