Re: Problem with shiho nage :( please help!
Just to reinforce the safety issue:
Part of the fun of learning techniques is playing around with them, but I agree with the idea that shihonage in particular can be a bit dangerous if either or both of the people practicing it aren't familiar with it. Compared to some other techniques, it can be easy to smash the back of the head, which is potentially lethal, never mind the more likely risks of lesser head trauma or damage to the elbow and shoulder if either of you should lose your balance (or your uke panic at the sudden loss of control combined with feeling of his/her head falling backward) at the wrong time. Part of what can cause injury is uke tightening up and resisting the technique in the wrong way.
I'm guessing it's the issue Chris demonstrated, because that's a commonly described problem, but diagnosing (and fixing) the problem(s) should be done in person...especially for techniques like this one, in my low-level opinion. I'm all for talking about techniques and practice tips, even though I think those things are done best on the mat, but not knowing either of your skill levels I hesitate to offer what little advice I could give on the technique itself.