By looking at it, I said his will, because Kamui is from MS Eye and eye is connected to brain, and when brain process a thought to phasing on dimension, it will activated at will.
Good question. It's hard to answer, because at times it's seemingly impossible to have him dodge some of the stuff that he dodges by reflexes alone. But at the same time, he got hit by Minato's Rasengan (which implies reflexes).... So... I'd say it's his reflexes based off of those since it's the most logical answer.
But... Again... Some of the stuff that he has dodged via kamui just doesn't make any sense for reflexes alone.
Then how is it a counter to limbo?
Speed of thought is faster than limbo I think. Its enough.
But if you cant see or sense it how would know when to activate it?
Kamui phasing is passive. He doesn't have to choose when and where it is used.You must be registered for see links
He was hit by Minato's rasengan because he deactivated Kamui in order to try and suck him in.
No one can react to limbo if Rikudou chakra not being used or not present inside body.
Kamui phasing is not automatic and it not having a sensor, hut Kamui activation is automatic or at users will.
Because that Madara want to used Limbo, but Obito is having rinnegan is a risky for him, but he is not aware that Obito also possessing Rikudou Chakra.
Obito's doesn't have Rikudou sensing, but with the rinnegan he can see it.