[acdha]

That last part is key: the MCAS system was designed to fake handling like the older planes but they skimped on safety to save the cost of a second sensor and didn’t train pilots on it or have an override mechanism. If the whole thing had been aboveboard they’d have saved so many lives…

[exidy]

There was an override system, MCAS drove the stabiliser trim motors and so flipping the stabiliser trim motor cutout switches would disable MCAS. This relied on the pilots diagnosing an MCAS runaway as a stabiliser trim runaway and enacting the same checklist.

However, to add insult to injury, the MAX also had another change. In the 737 NG, there were two switches, one would disable automated movement of stabiliser trim, the other would cutout the electric trim motors entirely. This allowed the pilots to disable automation without losing the ability to trim the aircraft using the switches on the yoke.

The MAX changed this arrangement, now either switch would cut power to electric trim. Tragically the pilots of Ethiopian Airlines Flight 302 recognised the runaway, enacted the correct checklist, but the aircraft was now so far out of trim that aerodynamic loads made correcting the situation using the hand trim crank impossible. In desperation the pilots restored electrical power to the trim motors, MCAS re-engaged and drove the aircraft into the ground.