A full auto rifle has a disconnect that allows it to fire every time the bolt slams shut.
You've got it backwards. It's a common mistake, along with some others, such as, I've had a number of people who thought they were informed, but weren't, tell me that "all you have to do to semi to make it full auto is file the shear pin"
Semi autos, (and some designs of manually operated repeaters) have a "disconnector" a part, or parts (that full autos don't have) specifically designed to PREVENT full auto fire. They "disconnect" the trigger mechanism, such that it requires the trigger to be released (to a certain point) allowing the parts to reset, so that another pull of the trigger will fire the weapon, again.
This ALSO performs the function of not allowing the gun to fire until the action is fully closed (and locked, if that's part of the design), as the trigger group parts won't reset until the action is closed. They are "disconnected" while the action is unlocked (and open).
Specific to "bump firing", the ATF got it right, during the Obama administration, when they ruled bump fire stocks are not full auto, because, quite simply, the trigger is being "pulled" individually for each round fired.
The "pull" of the trigger with bump fire is actually a push, but the effect is the same. The rifle recoils back, away from your trigger finger, which is held in a stationary position, and something (you, or the stock) applies forward pressure on the rifle at the same time, and being held loosely enough, the rifle will "bounce" back forward, "bumping" the trigger against your stationary finger, firing the rifle.
This meets the legal definition, of "pulling" the trigger for each individual shot. Therefore, NOT a full automatic, as defined in law.
When the ATF "re-examined" their ruling, the ADDED language to the regulatory definition (they couldn't change the language used in the actual law) and THEN determined bump stocks could be regulated, because they "initiated a continuous firing cycle with a single pull of the trigger".
This is a problem on two levels, one being that EVERY firing, both semi and full auto is initiated with a single pull of the trigger, and second, it completely ignores the earlier finding (and reality) that the trigger is still being "pulled" once, per shot. Smoke and mirrors, my friends...