I think you hit the nail on the head. Because the tenkara rod & line are short and the perceived need to get every inch of distance I do extend my arm at the end of the casting stroke.
I'm not sure if I can make a nice abrupt stop with the Hera rod as it's 18 ft. long and has quite a bit of leverage on my arm & hand. But then if I can't do it one handed two might do.