Tenkan in Aikido is( again this is a very crude explanation!) a 180 degree turn. Tori (the person being attacked) absorbs uke's (the attacker) attack.
It was like Nishio sensei said in the Irimi Nage clip: In Aikido, the force isn't blocked or stopped, its taken in a different way i.e. redirected.
In Tenkan, the movement is taken the direction it is going then redirected. In the below example, Yamada Sensei shows the Tenkan at around 00:10. Uke (attacker) is moving his left arm forward. That is the direction of the attack and his movement. So Yamada Sensei uses Tenkan to keep that movement going (while avoiding the attack) and then using that movement to re-direct the attack. Hope that makes sense!http://www.youtube.com/watch?v=Wgc25csUSMA
On a non-Aikido note, found another clip, don't know if this is what you are referring to (sorry I don't know much about karate!):
Keinosuke Eneoda, Shotokanhttp://www.youtube.com/watch?v=v38XQJgK7W0