The best nikyos I've felt induced collapse. There was no pain, just a switch flicking from off to on, and I went from stood up to not. Nikyo seems to present the body with a moment of peak pressure that convinces your body to try and get lower/underneath it. Pain isn't part of the equation, unless the duration increases and the pressure decreases. I suppose this could be done accidentally or intentionally. A lot of beginners have nikyos that cause a fair amount of pain, but don't take your balance.
The question is whether that "induced collapse" is a developed "Pavlovian" response to having enough done, um, with "feeling" prior. How many have shown a brand new student a nikkyo only to have them stare at you? In pain. Eyes watering. But completely oblivious to how to go down and relieve the pressure on their wrist.
In other words, an experience person manipulating your structure through the wrist such that you get the "right" feeling meaning you take the "proper" ukemi at that point. The question is whether you "really" had to go down due to the structural feel *or* if you simply feel *both* the structural manipulation ("correct" feeling) and you simultaneously realize the next feeling coming is going to be the pain. then eventually you know the structural feeling and react to that prior to actually having to move? Conditioned responses are by definition automatic and rather instantaneous and feel like they were not intentional.