I've been giving this a little thought, and I believe it's may be what happens at the point when uke is helpless to resist a technique that dictates a hard or soft style.
First of all I would have to say that for a technique to work efficiently, it should take little strength or power.
That being said, after the off balance/redirection/leading or whatever you do, there is a point in time when uke is at a distinct disadvantage.
i.e. balance is broke bad posture, etc. and tori is at a great advantage i.e. wrist lock etc.
after you have uke in this position, you can break his arm, or gently place him on the floor.
maybe this is what makes the style soft or hard, depending on the attitude tori takes.