If the bolt is open, it closes the bolt.
If the bolt is closed, the button puts a shell on the lifter.
This is why in early M1’s, you can “ghost load” an extra round….you load the mag tube to full capacity; retract the bolt back far enough to insert a shell, but not lock back; put a shell on the lifter and push it down, then put a shell in the chamber and let the bolt go forward.
It makes for an insanely fast second shot; using slugs or magnum loads, I have bump-fired a controlled pair with a timed split of .08; this is basically going cyclic.