Loose rivets, OK - been reading a lot more.
The speed target for STS appears to be either a previously set speed or the speed the plane was going when a pilot last let off the trim switch - sounds like an action similar to cruise control in a car. Lock it in at a set speed, but if I trim that speed up or down, the cruise control uses that new speed. However, it's smarter in that one of the problems is needing to handle the undamped phugoid which it does by reacting more quickly than the natural oscillation of the plane.
From the 737 page
Flight Controls :
Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas, pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition.
Per
B-737 Speed Trim System
By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.
From that thread - it was to solve the problem that at aft CG and high thrust there isn't enough trim reaction force to meet the minimum gradient of 3 pounds per 1 Degree AoA change. If the CG was at the Center of Pressure no stick force is required for any AoA change - hence this moves the trim opposite to the pilot input as the CG approaches that (hopefully unreached) condition. That is, if the pilot pulls back to slow the plane the STS supplies nose down trim to encourage the pilot to speed it back up.
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It appears the effect of STS should have been to push the nose up as the plane accelerated and MCAS was pushing the nose down; the opposite. While STS doesn't move to relieve trim loads, it moves to reset the speed to where the trim load is zero unless the pilot is pulling or pushing.