In your example (the Marlin action) the lifter (aka carrier) also functions as the shell stop.
There are cam surfaces on the lever that interface with the lifter. Cycling the lever effects up and down movement of the carrier, the front of which blocks cartridges from sliding back onto the carrier except at one precise point in the cycle.
It's kind of hard to explain, but if you google "the Marlin jam" you'll find some better explanations of it.