Re: Is aiki a clash of forces?
Tim knows how to fully put his weight down, into whatever spot he wants. He's not holding any undue tension, so all of his weight can go where he wants to direct it. If that's down, he feels super heavy, if it's out he hit's or throws you super hard.
You lose your balance for any number of reasons: You were stuck to him and moving him unbalances you. You use more pressure then needed to try and move him, you over extend. He puts something (leg, foot, hip, etc) in your way when he moved so you fall over it. He directs your movement when he moves so you're in an awkward position after he moves, you fall down with no effort on his part.
There are lot's of ways to train it. Some of it comes from organizational exercises (ways to use your body) some of it comes from hours and hours of sparring, and understanding how certain things will play out.