Why Uke's can be wrong...
I wanted to throw my two cents in here since this issue presents itself every class. Please forgive me if this has been covered as I only read the first ten posts and my eyes started burning. Getting past forty is trickier than redirecting a committed uke!
I have found that many of the people who take Aikido as their first art (unlike many of you who have solid Karatedo, fighting backgrounds as I do) tend to be more passive than the average person who trains in the martial arts. Personally I have been amazed that these people stay in my classes, as I am an old style Aikido instructor. Not quite Saito Sensei style (more straight line than circular) but harder in the fact that my Iaido and Karatedo backgrounds and the bushido mindset seem to find their way into my techinques. I have long considered that learning Ukemi is an art within itself and an essential part of understanding to be an effective uke. Therefore in order for my "newbies" must learn to understand and give an effective attack to be able to be a good training/ rondori partner.
It is an interesting transition for the passive individual to undertake, and seems to be extremely rewarding when they hit Gokyu and things start to come together. Not only do they gain the ability to effectively redirect a strong and committed attack they can deliver one if needed.
Which brings me back to why the Uke can be wrong. Basically it comes down to commitment. As O'Sensei has been quoted in many Aikido publications: "life and death can be defined in a single technique". It took my maturation in Iaido to understand this. Without the uke's intent to harm present in the attack the strength of Aikido cannot be realized. Without the single-minded focus of the uke on the attack the nage can be ill prepared to blend with the technique. Many young Aikido instructors have taken a shomen-uchi on the head or a chudan tsuki in the gut by a strong uke. I am not one to think that the uke should alter his attack just so the instructor saves face. Many think this as disrespectful and they are fully entitled to do so. If one of my students "pulls" and attack out of respect I thank them and then ask them follow through next time.
This commitment can be achieved at the early stages of development by slowing down the speed but not the intensity of the attack. A chudan tsuki at 10% speed won't hurt badly but it should hurt a little. When the nage gains efficiency in their timing the speed increases until a full blast attack is a nage's best friend. Not everyone will attack like that but if a prepared Aikidoka meets a life or death attack the speed and power should resemble just another training session.
That's just my take on it. Hope you enjoyed it.