I have a training partner like this, who analyses the technique he knows is coming and figures out how to thwart it before starting his attack. He thinks he is being realistic by providing a committed attack and resistance to the technique, but in truth he is training us both for a situation in which the attacker can read the defender's mind, obviously not a reaslistic situation.
I assume this person is your sempai. Often from my own experience when this is the case I have found that the best thing to do is to copy exactly how they do the technique. It is not that they are necessarily trying to stop you from doing the technique, but that they have a preconceived notion of how they think it should be done. By coping them it will placate them and they wont resist you doing the technique the way they believe it should be done, otherwise they are completely hypocritical and ridiculous. You also learn a slightly different way of getting around power, and learn a new way to do the technique, all valuable stuff.