If you focus on the drawing in hand instead of the striking hand, wouldn't that free up the entire body to do the strike, instead of clenching up? Endo shihan has a segment in a recent DVD where he discusses moving when uke has you in a strong morote-dori. He said the important thing is to move the free hand and that will open up the rest of the body.
Purposeful intention in your body will definitely cause your body to tense-up (besides your fist). Physically and "mentally" releasing intention not only "frees up" your body, but the impact is even more profound upon the uke. I would go so far as to say that the nature of this type of "mind-body" movement applies to how we respond to any and all attacks. George Ledyard speaks very clearly about the importance of the mind (mind-body) entering into the other person's center before any movement takes place. The really difficult part is using this "mind" without intention.
Tonight, I had my students focus in on the differences (both nage and uke) when using one's intention in movement and letting go of intention in one's movement. This focus was on a very simple movement. When students (I ALWAYS include myself in this category) can feel how difficult this is to achieve as the intensity of the attack increases, students can gain some sense of the true depth of this topic. More importantly, the students gain a sense of real power in executing techniques.