Solo, You only hold it a tad bit shorter, but alternating every now and then is a good idea so he isn't harder in one side of the mouth than the other
Hrt4dressage.... Sorry, I never explained but I don't do this softening exercis until AFTER the one you described. I make sure that they are very light in flexing laterally both ways and that they completely understand how to give to the bit.
After that then I do this exercise I described because they understand how to give to bit pressure. And yes they do round their backs and put more weight on their hindquarters. Most of this is because you have to "arch" their back with your legs a bit and keep them going and on the bit.
Glad you said that though. It gave me a chance to clear things up. It's all in my head, but I sometimes skip things when I post.