Quote Originally Posted by DB_Hunter View Post
Ah Ok.

Kakashi developing the MS at such an early age does kind of throw his development off a bit. Having such an ability yet going through such dangerous situations as the fight with Zabuza, Orochimaru and of course Itachi without zapping them away in to another dimension whilst risking life and limb is a bit hard to explain away. The only reason I can think of is that he felt his MS was a result of killing Rin and as that was something he hated he didn't want to benefit from having committed such an awful action. However when it came to fighting Itachi he realised the only the way to defeat him and Akatsuki was to use MS which why he began to train using Kamui.
I'm thinking its more or less he didnt have control over it and would pass out if he even attempted to use it. After the skip while him and Naruto were chasing Diedra, Kakashi uses it then, mentioning that he's been working on something new. Throughout the time, it seems every time he is able to use it a little bit more and more.

My 2 cents would be similar to SSJ stuff where every time they come close to death, they get stronger. So every time he uses it and is able to rest, he can use it more and more.