Originally posted by Greg Jennings
There is a saying among the long-time e-aikidoists. "Aikido works. Mine might not".
Actually, I think the original quote goes something like, "Aikido works. Your
aikido doesn't work. Please don't confuse the two."
I guarantee that my instructor will give you shihonage that you can't "just let go" of.
I think there are a few factors we have to think about here.
If a person is grabbing your wrist, it's usually not so they're going to let go immediately but to do something, whether it be to break your balance, immobilize you, keep the grabbed hand busy so they can punch with the other hand, or so on. Because of this, nage will usually have a split second to do something before the next "action" (whether it be a push, pull, or a punch) occurs. During that split second, nage has to take the initiative (sente) to get rid of uke's ability to effectively do anything else. This may be anything from a pushing into uke's hand so they can't escape, rotating your wrist so as to lock their elbow/shoulders, breaking their balance so they can't but grab onto your wrist for support, and so on.
Many different dojo have many different approaches to make sure that the techniques themselves do, in fact, work.
But, yes -- I agree with Greg that shihonage, when done correctly, will not let uke "just let go."