I'm not entirely impressed with the UM torque sensor, I mostly rely on my (derestricted) throttle to get smoother speeds and full power when I want it.
For example if I want a leisurely ride along the flat it's difficult with just the torque sensor to go slowly smoothly (ie below the 15mph motor cut-off), putting it in 6th gear and pedalling slowly the motor will cut in and out as my speed decreases and subsequent pressure on the pedals triggers the torque sensor. In a much lower gear the torque sensor wants to kick in much more because it's easier to put more pressure on the pedals.
I got to have a look inside one of the UM VPAC's when I had my one replaced (the sensor still worked but the chainring guard was smashed), it uses a similar system to an old mouse that uses a ball against a rotary encoder, when you press down hard on the pedals the inner ring moves independant to the main chainring and this causes gaps to appear in which the infra-red sensors get triggered.
It's quite weird first getting used to the torque sensor because of the initial give it has before it gets to the stopping point and then the power is transferred directly to the chainring. Check out The Gadget Show clip on UM's site see what I mean -
Urban Mover - the future is now (I got to ride that actual bike
)
Really they could do with some way of measuring not only how much force is put into pedalling but that you're actually pedalling and how fast, to then throw the numbers into some magic calculator to get a smooth ride at any speed up to 15mph and not have the bike always try to get to 15mph when you're pedalling.
They do have a new thing on their new controllers that allows you to set how much assistance in percentage (25%,50%,100% I think) you get when pedalling which might give you a smoother ride, a longer one for sure.