[phire]

MCAS can't really be disabled.

They procedure just disables the electric motor that MCAS is using to adjust the trim down. MCAS will still be sitting there sending trim down commands that get ignored.

Problem is, that electric motor is used by the manual trim adjust buttons too, so they stop functioning too. Following the procedure results in a nose down trim that pilots have to correct by manually turning the trim wheel, and it turns out the aerodynamic forces may be too strong for that procedure.