Sounds like a very interesting exercise. We do a similar exercise in kokyudosa. Uke grabs ryotedori on nage and then extends. Nage has to stop Uke's extension by blending. Sometimes Sensei gets the yudanshas together so they can train together. In that situation we are expected to do kaishiwaza(sp?) if Nage has not done the technique correctly.
Some of training requires Uke to have a very directed attack where the direction of the energy and the intent from the attack is very clear. Nage has to blend in order to perform the technique.
Other times the intention o the attack is masked and Nage has to understand and be sensitive to the weaknesses in posture of Uke.
Happy training all