Any technique can be stopped if uke knows what's coming and knows he won't get hit.
I hear or read this sentence from time to time. But I have to admit, that I don't understand it.
Because in the practice I know, one of different ways of keiko is to resist, to block, to try to stopp the technique of tori. Methods can be sophisticated like being rooted and centered e.g.. Or they can be raw like using just muscular force. Can be smart like changing body position and posture or repositioning the feet. Or can be dumb like ignoring atemi completely. Kind of exaggerated go no keiko.
Ue does all this knowing what's coming and knowing he won't get hit. Tori has to find his way. Not by muscling, not by using something uke gives. It feels a lot like a "pushtest in motion".
Sure, you may ask whether it is usefull to try to bring the arm of uke up, while uke is pressing his arm forcefully down. It would make more sense to just use his movement and power ...
But with regard to technical exploration and development it is very interesting to try and to learn to move uke the way you want instead of moving him the way he inidcates.
For me at least it is an aim of my practice to learn that a technique can not be stopped although "uke knows what's coming and knows he won't get hit".
But I think it is also true, that there is no way a higher skilled student can be thrown by a not so skilled student, except he allows to be thrown. To me actually this exactly is one central aspect of the meaning of "higher skilled". Years ago, when I realised that my teacher couldn't throw me when I didn't want to be thrown, this was one reason among others to leave his dōjō and find a new teacher.
Beining a good uke actually is a great challenge, I think. To find a good way between not to just tank and not to be unmoveable. But being really "responsive", really giving a feedack to tori.