"Strictly it's not a new stall warning system, it's an anti-stall feature that trims the aircraft nose down to prevent it stalling."
You can describe MCAS like that, but the reality is the 737MAX is unsaleable without a trustworthy MCAS - it would either need aircraft recertification or crew retraining or both, in which case... no sale.
More info than I can be bothered summarising:
"When Boeing set out to develop the 737 Max, engineers had to find a way to fit a much larger and more-fuel efficient engine under the wing of the single-aisle jet’s notoriously low-riding landing gear. By moving the engine slightly forward and higher up and extending the nose landing gear by eight inches, Boeing eked another 14% improvement in fuel consumption out of the continually tweaked airliner.
That changed, ever so slightly, how the jet handled in certain situations. The relocated engines and their refined nacelle shape caused an upward pitching moment — in essence, the Max’s nose was getting nudged skyward. Boeing quietly added a new system “to compensate for some unique aircraft handling characteristics during it’s (sic) Part 25 certification” and help pilots bring the nose down in the event the jet’s angle of attack drifted too high when flying manually, putting the aircraft at risk of stalling, according to a series of questions and answers provided to pilots at Southwest Airlines, the largest 737 Max operator reviewed by The Air Current.
The Maneuvering Characteristics Augmentation System (MCAS) was designed to address this, according to Boeing engineers and pilots briefed on the system, now at the center of the inquiry into the crash of Lion Air 610, a brand new Boeing 737 Max 8. MCAS is “activated without pilot input” and “commands nose down stabilizer to enhance pitch characteristics during step turns with elevated load factors and during flaps up flight at airspeeds approaching stall.”
[continues - please refer to original article]"