The dynamic defenses against grip attacks share a common curse when considering them against real life encouters - the attacker must hold on in order for them to work. Taking it as a given that the attacker will not
hold on, whatever the methodology of the defense; what good are they then?
Before that though: Why do I say the attacker won't
hold on? 2 reasons; one psychological, one tactical. Psyche reason: Keep in mind if a person's attacking; he'll be in attack mode - his mind and body committed to the act of doing violence. By committing the attack; he's already taken the initiative in the engagement and will strive to keep it. This is where a RL attack differs from uke performance: uke doesn't want to hurt either nage or himself. An RL attacker does
want to hurt the defender, that is in fact his goal - and the increased responses and ability adrenaline provides limits or completely negates the possibility of himself getting hurt by letting go. Short form: The moment he feels something is less than ideal; he'll drop his attack and switch to something else.
Tactical reason: A dynamic defense from a wrist attack depends on the attacker not letting go; as stated above; so ignoring all else; there's a 50-50 chance of that happening - either he'll let go or he won't. Betting on an even chance is a losing proposition; in an RL encounter it's best to assume he will
let go - plan for the worse contingency.
OK - that out of the way; back to the dynamic defense and whether or not it'll work.
It won't. It's not designed for that.
Most people seem to think that techniques are practiced as they are because that's how we'll do them in RL. The techniques are not
useful for the 'street'; their value is far, far greater than that.
A friend (by correspondence) of mine compares using the MA for self defense to wallpapering your study with artwork from the Louvre - it'll work; but the power and grandeur of such fine art is rendered meaningless.
See; techniques such as kokyunage (variants of which are the dynamic wrist defense we're discussing) aren't really for real life - their true purpose is to teach the practicioner. As Ian so excellently discribed; the techniques teach the body movements, blending and above all mental conditioning to use aikido - they're not in themselves aikido; they're the aids to learning it. They teach body geometry, kinesiology, the physics of two bodies in particular motion. If you understand what the techniques are teaching; you can recognize a given situation when it occurs and act accordingly.
I'll use katate-tori ikkyo as an example. Ikkyo is probably the most effective technique in aikido; but an attacker will never
grab your wrist and stand just so while you go through the technique. However; if you study it long and well enough; in a defense situation when in whatver way the attacker's wrist and elbow come into position; you'll be able to put on ikkyo without thinking. The goal is to use aikido
; not technique, IMO.
So my advice would be not to concentrate on 'whether it'll work'; concentrate on learning the technique as well as you possibly can; and study what the technique is teaching you.