A near-perfect example of the seize&freeze pheomenon that I've been talking about this week is hizaguruma (knee wheel) in judo. If tori gets you into position for hizaguruma and you straighten your back or pull back against it, then your low back and hip muscles lock up (seize&freeze) and you break and turn over in the air pretty abruptly. However, if you get into hizaguruma and instead of seizing and freezing, you relax your lower back - even to the point of slouching forward or laying your head on tori's shoulder, then your relaxed back and hip muscles facilitate a more complete range of motion and more often than not, you can pick up that leg and walk out of the unbalance. It is uke's seize&freeze reflex that makes hizaguruma into a large, magnificient ippon. (See can't you find a YouTube video of a hizaguruma that stalls for an instant, then suddenly breaks and smashes uke. I bet you can)
The same applies to deashibarai (front footsweep) in judo. This throw most often results in a nice, low-amplitude sidefall for uke, but every so often someone does deashibarai and actually clears both of uke's feet into the air and uke drops like he was shot. Times like this, deashi is not uke's friend. So, how does someone end up getting both of uke's feet into the air when only sweeping the front foot? That's right - seize&freeze! Uke's foot starts slipping and he naturally resists by tightening up on his hip muscles. These suddenly-locked-up muscles bind the hips and both thighs into one big lump of meat and bone and the standing foot starts to slip and rotate along with the swept foot. Seize&freeze has killed uke again.
When you get right down to it, I suspect that seize and freeze plays a major role in nearly every fall we take. Take shomenate (face strike) - the most foundational thing in aikido - for example. This time, tori deliberately locks uke's spine and back by pushing upward on uke's chin until he is looking straight upward. All of a sudden uke's entire torso is locked into one big chunk and his hip and leg range of motion is limited. At this point, tori drops his entire weight forward through uke, throwing uke's locked-up body backwards and downward. Shomenate done with proper spine-lock is a very severe thing. Shomenate done without full spine lock usually results in uke fading back and absorbing or countering the throw.
I bet once you get to looking for it, you'll find lots of examples of this reflex being the difference between you getting a marvelous ippon and uke shrugging off your throws.