i dont get obito's kamui?

[ShiroT]

He didn't know where Gai was going to attack him so he fully materialized to avoid any risks.​

if he did he wouldve fell through the 8 tails in like in the other scan he couldve grabbed naruto's face and cared him off but im sure he slipped through him

 

[Captain Incognito]

Remember Gaaras automatic defence when he was still a Jinchuuriki?
Well it's almost the same with Obito and Kamui.
His intangibility works like this, Obito sees or senses an incoming attack, during the incoming attack his whole body is in a state of caution, it still exists in the physical realm and it still has physical properties, however during contact the affected body part loses it's physical properties and is send away to the Kamui dimension.
Sure he does have the sharingan and he's able to predict movements to some extent, but he still needs sometime to react and even if it's just a brief second, that's just enough of an opening for some people in the NV.
Where as Kamui or his intangibility, to be more specific, is instant.

 

[Oblivionx]

He can attack while using kamui on parts of his body:

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Plus this scan further explains it more.

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He can attack while using kamui, he just cannot absorb himself or someone else while using kamui, he needs to be not using kamui to be able to absorb anything.​

this guy explianed it best....!

 

[Saroku]

Kamui isn't that superior, it can't differ between an arm, a leg or a head, the only thing it can is to transfer anything away that is about to be touched by an enemy.
And no, Obito didn't use Kamui while attacking Kakashi after going trough the wall, he was already out of it.
The second picture says that the part that overlapps with the enemys body is transfered away. Not the part Obito chooses.
You guys should really read the manga more carefully:

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Really, what you guys are saying is like saying that Pain can use Shinra Tensei with no 5 second interval.

 

[Draphsin]

Kamui has two modes, on & off. While on obito is completely intangible & can't be touched for 5 minutes no matter what, & while off obito is solid & can't phase through anything.

That's why even if his foot is in the dimension he still can't use his hand, because his hand would phase as well.