Oh yeah, that was interesting. They were renaming it to be part of the Speed Trim System, with the MCAS name used only internally. After doing some looking, apparently the STS is a system on all 737s that adjusts the stabilizer in order to increase the stick force. This is necessary to ensure the plane meets the FAR speed stability requirement for the minimum increase in stick force as the plane moves away from the current speed. It does this by moving the stabilizer in a way that opposes the flight stick input, and is active 5 seconds after the last manual trim input when autopilot is off. If you've been following the discussion of MCAS, all of those details are probably feeling awfully familiar...