At our dojo, as uke we are taught to act as mirrors of our nage...to try to accurately reflect the condition of his mind and body coordination. By extending Ki (which is our form of resisting), the nage should be able to find the holes in the technique being learned. I think this is most beneficial if both the uke and nage is approximately at the same skill level as they could continue to challenge their partner to the next level.
I agree that advanced students and sensei should offer the appropriate level of "challenge" to their students.
Not to pick on you, but this is exactly why I have no idea what is expected of me as uke. What would you want from me? A single committed attack? Should I attempt to use my ki to prevent myself from being moved (rooted?), should I use my ki to prevent my arm from being moved or bent? Do I flow with nage and move where he wants me to go? Do I keep focused on my center and attempt to throw solid committed attacks without giving my balance away? Do I blend with nage and use his movements to guide him into my own reversal?
I have heard so many different things ment by 'extend ki' that I have no idea what would be the desired goal in the context of a drill. This is in stark contrast to the very detailed instructions given to me in sport training (attempt to take him down, try to hit him in the face, throw punches at 50 percent speed at his chin, block 3 jabs, throw 3 jabs, try to clinch, attempt to hold down your partner in side control while he tries to escape, etc).
I have been told to extend ki in the context of a ki test (unbendable arm, unmovable, un-liftable, etc), in the context of a strike (focus though the attacker like a laser beam at the wall across the room), as a push (move from the center), as a means of balance, etc. I could attempt any one of those things, or all of them, or something else. I could throw a single strike/grab, I could free form attack, I could really try to hit my partner, I could throw a non threatening blow that would never hit, and still meet the types of training in which I have been told I was extending ki.
I guess these kind of explanations (use more ki, etc) just stonewall me. I am a scientific learner. I like exacting responses (keep your hands up, you are flat footed, you did not take his balance first, etc) and exacting requirements from me (punch him in the face as fast as you can, grab and attempt to lift your partner off the ground, throw harai goshi 10 times as fast as you can, take this guy down and submit him, etc). Every time I show up for aikido class, I have no idea what is acceptable and what is unacceptable, every time I ask I am made to feel like a smart ass thug who wants to fight (which is questionable).