There really is no sense of force. I can't find the term 'irimi' associated with karate on the forums (yet). I suppose this video touches on it.

http://www.youtube.com/watch?v=yrYKn7Rtkwg

What I can understand of this mechanic so far, is it's part of blocking, unbalancing, and redirecting the other person...they just go flying if you get the angles right.