Yes Neil the solution to the not being able to back off the throttle is a Half hand throttle or thumb throttle. http://www.youtube.com/watch?v=f9fQ2ewxHQg
^ This reminds you of anything?, LOL
Ive flipped a one or two bikes upside down with the full hand throttle myself, and wife included. Its not a pretty public spectical down town around people.
The reason being, the bike pulls (or pushes in the case of the above clip) on your arm, wrist and your hand, and its wrist against motor. The more the bike moves forward, the more it pulls on your arm and wrist, and adds to the twist, the more power the motor gets. Peoples nervous systems are wired in panic mode, and they can not just let go, no matter how sensible it would seem. Its just how it is. The brain can not intervene when your automated nerivous system "grab mode" when the "hold on for dear life" function is in process.
Unfortunatly, if you panic, which every one does to keep the bike under controll, the motor always wins every time.
Why half hand throttle totally solves this issue, and in many cases, even in situations when youre not showing off; (EG. maybe walking the bike using the motor softly to assist), is that half hand throttles provides the other half of your hand something that doesnt twist with the increasing forward movement of the bike. IMO All motor bikes and Ebikes safety could benefit from the use of haldhand throttles.
I had my old SLA beast fall over and damage the throttle, and on the way home the whole twst grip came off in my hand leaveing the hall piece on the handle bars. I had no brake switches installed and the bike went into full speed. like a runaway train. I had to manage to plug the twist grip back into place inorder to operate the bike. Half hand thottles resolve this issue too.