Oops I was forgetting...
If I manage, I don't mean stop, but create some resistance for Sensei it is not only useful for him, but for me too.
As a Uke I would have the chance to feel on me how Sensei adjusts to that new situation, how he moves his body, how he changes his arm movements and so on.
It happened to me last Saturday during a kokyunage. As long as my Uke was attacking my center was perfect, but when his attack was a bit deeper I was having a lot of trouble. I was the one who asked him please to attack deeper and not to go down easily so that I had to work a way to get to his center.
That doesn't mean he had to turn in a piece of wood and get hurt. He still was flowing with the technique and put his safety first, but he was providing me what I was needing in that moment to work on mostly. Had he gone on and offer me an attack that didn't give me any trouble I would have never known how hard was for me to do that technique in another situation.