Three steps: bind, enter, cut

The heavy chain of the toxin binds SV2 on the surface of a cholinergic nerve ending and is swallowed into a vesicle. Acid inside that vesicle changes its shape and threads the light chain out into the nerve's own fluid. The light chain is a zinc protease, and it cuts SNAP-25, one of the three SNARE proteins that zip a vesicle onto the membrane to release acetylcholine. With SNAP-25 cut, the vesicle cannot fuse, the signal cannot cross, and the muscle fibre stops receiving its order to contract.