most painful throw/RE.
That was too silly a response ... seriously.
Some people have very loose joints, from practice, or just as a natural body trait. In order to properly deliver a joint lock, you either have to learn to feel the tightness of a twist, or add a base to extend the stretch of the joint, securing complacentcy of your partner ... plus giving them the opportunity to feel what you feel, pain. This is not being mean, but you should be versed in the variety of tensions needed to make techniques work! (Although, sometimes we are too kind in Aikido, that we even throw ourselves to avoid pain and stay ahead of a technique?)
Whether it is moving your hand to use the fingers, or parts of the hand to create the base, or simply add a twist to the bend, you must study these things to effectively create the pain, also known a opening/diversion, to continue to properly protect yourself and give your partner the opportunity to experience protecting themselves while looking for the openings in your technique.
Once you start to understand the variety of tensions it takes to create pain, the use of proper body movement, the hips / not the arms or upper body, you have to be even more careful as injuries come that much quicker than they did with improper movement.
Sometimes the pain the most painful throw is inatttention by one of the partners to blend with the movements and technique of the other ... that is presupposing they haven't gone too far and entered the injury zone of pain?
I was there a couple of times? All I remember is the cool breeze, the wave of sleep, then smelling salts with people asking, "... are you alright"
That would be a NO! I passed out didn't I?