I'm posting for no other reason than to help those begining on their martial arts quest. I'm not particulary interested in thread ratings, "experts" from Japan, or any other Aikidoka who spend any amount of time griping. Disagree is fine. Disagree with a comment is better. Order me as if I am a newbie, worse.
If the information I present has been discussed on another thread, I am sorry. But then since it must have value, discussing it again will be beneficial.
Momentum is one of the manifests of ki. You can eventually feel an attacker's power coming at you. Simply blend with the ki until you get the opponents center/ki. (To test this, if a kotegaeshi is applied slowly enough, one will feel a threshold where the opponents center/ki is captured. He/she will be unable to move at all.)
Build that feeling add extentsion, softness, circles, and sensitivity and you've learned yourself some martial arts!