More complex answer; Its the amount of recoverable energy. There's more than 2x as much "available" energy at 40 MPH vs. 20 MPH. In addition depending on what gear the driver may have engaged the engine could be more easily stalled (particularly in 5th gear) should regen be "pushed" to lower MPH's. And finally there's probably a point that pushing any more charge over time adds undesirable heat to the batteries. So given all factors a good design choice is to decide on an absolute limit that addresses all three.
Mine works below 20 MPH. I'm not sure at what speed it does cut off, but it definetely regenerates below 20 MPH in 1st or 2nd gear. I will try to remember tomorrow to check and see how low the speed goes before cutting off.
During deceleration, the gasoline engine is switched into fuel cut mode, and the IMA Motor/Generator is operated in generation mode. In this mode, the IMA Motor is driven by the wheels, generating electricity to be stored in the Battery Module, and slowing the Insight in the process. The amount that the IMA Motor slows the car is in proportion to the amount of regeneration being done. There are two deceleration modes:
Foot off throttle but not on brake pedal - In this mode, the charge/assist gauge will show partial charge, and the vehicle will slow down gradually.
Foot on brake pedal - In this mode, a higher amount of regeneration will be allowed, and the vehicle will slow more rapidly. During light brake pedal application, only the IMA Motor/Generator is slowing the car. With heavier brake pedal application, the conventional friction brakes also come into play.
When decelerating, regeneration will continue until engine speed falls to about 1000 rpm. At this point, the driver will typically shift into neutral. In many cases the gasoline engine will now immediately enter auto idle stop mode. If vehicle speed is such that it isn't clear whether the driver will most likely come to a stop, the engine may idle for a few moments before entering idle stop. If the battery state of charge is very low, idle stop will not be entered at all, and instead the engine will continue to run at a fast idle to recharge the Battery Module.