This would be a great place for George Ledyard Sensei to chime in. I had the privilege of training with him at the Aiki Expo '05. We were doing kata dori sumi otoshi and I simply wasn't getting it. Ledyard Sensei was like trying to push a Buick! He was, by deftinition, resisting me. After a couple of tries, he corrected what I was doing and then dropped like a rock. Obviously he could have countered anything I was doing, but he gently resisted my technique and then showed me how it was supposed to work. I was grateful to him and remain so. Maybe he can explain if, when and how resistance can be a good training tool.