He should not be able to do this if his balance is broken. You do not have him unbalanced which happens at the beginning of Shihonage.
I would be more concerned with unbalancing your uke than muscling him.
I did not extend him to unbalance him as I should..I flowed into the attack as if he was moving...and since he was static it did not have the desired effect..I guess the whole lesson is simply this: I was concentrating more on moving and technique, whereas he was wholly concentrating on resisting.
Janet - thanks for the tip on the fingers ..I have seen/felt Sensei do that ...it is quite effective!