737m has two Flight Control Computers (FCC). Each FCC contains the control logic for the MCAS function. Each FCC's MCAS function uses the AOA vane signal from that side of the airplane to compute its MCAS command. When flying manually, only one FCC is active at a time (active channel switches each flight).
If one AOA vane signal as received by the FCCs is erroneously high, the stick shaker on that side of the airplane can activate erroneously. If the vane signal that is erroneously high happens to be the one feeding the active FCC, MCAS may activate erroneously. If the vane signal that is erroneously high happens to be the one feeding the non-active FCC, MCAS will be computing its commands based on the other, correct AOA signal and thus MCAS will behave as intended.