The 737 classic and NG has protection for avoiding exceedind Vmo/Mmo ,wing flap placard speed and landing gear placard speed. Also for not going bellow minimum speed (which some call alpha floor protection). Minimum speed is based on angle of attack and is aproximately 1.3Vs for the current flap configuration.
Those protection are available with at least one component of the AFS engaged or armed (ap or a/t).
when full manual you will only get warnings -stick shacker , overspeed clacker and MCP limiting symbols.
When ap pitch or autothrottle are on or armed ,they will engage and maintain speed in the protected area.
The reversion modes are used in case the selected control can not maintain any longer the speed in the safe area (for ex one select Vs of 6000'/min ,soon the speed will decrease approaching stall .When at 1.3Vs the modes will change and the ap will go to lvl chg ,reducing pitch so to achieve minimum speed + 5 kt.)
The same goes for the a/t .If armed,when sensing minimum speed it will kick in (very fast) and the speed will be maintained above minimum.
On the Bus , in the approach to stall regime, alpha protection limits the amount of pitch-up that can be commanded, thereby preventing too high an angle of attack (AOA). Should the situation deteriorate further in the approach to stall regime, the alpha floor protection also will apply take-off go-around thrust (TOGA).
On Boeing ,there is no limit of pitch (other than PLI on the EHSI ) ,but the AFS will compute max AOA and apply thrust if max AOA -,minimum speed is achieved.The a/t must be armed for this to happen.
I would like to know about the mode reversion traps you're talking about ,c100 driver.