I have always been told to push back into the rail slots when installing the rings. It makes it so that recoil does not have any room to push the rings backwards.
Recoil doesn't push the rings backwards.
It pushes the gun backwards.
Think about seat belts in a car. If you hit something, you don't stop, the car does, the seat belts prevent you from flying forward when the car stops.
The rail slots do the same thing as the seat belts, except they just drag the scope along with the rifle instead of letting the rifle recoil out from under the rings.
Attachments (like scope rings) will want to remain in place until acted upon by and outside force (gun moving backwards), so the front of the rail notch will be what actually shoves the ring backwards. It is probably possible to crank down hard enough to keep the ring from slipping forward, but it is much easier if you just push it forwards against the notch first and then lock it down.