To cut a long story short, they put massive engines on an airframe not designed for it, causing "unusual flight characteristics" (the huge engines could generat

To cut a long story short, they put massive engines on an airframe not designed for it, causing "unusual flight characteristics" (the huge engines could generate their own lift, pitching the nose up) which they compensated for with MCAS. MCAS took input from an AoA sensor, and if this sensor provided incorrect input the nose of the plane could be pitched down aggressively during normal flight - a very dangerous occurrence especially soon after takeoff. To make matters worse, it overrides pilot input (i.e. pulling up on the yoke doesn't help - the system has to be switched off). An AoA disagreement indicator, which may provide timely alert in such situations, was an optional paid extra (should have been a mandatory safety feature). Without this indicator and in an emergency situation, it might be too late to save the plane before the pilot figures out what is wrong and disconnects MCAS. Additionally, information about the MCAS was initially left out of the plane's manual. One could ask, in what way is it not preposterous? There's no way it should have been certified in that state.