So Obito can transfer any of his body parts that are in contact with the enemy to the time-space of the Kamui and make it look like he cannot be touched.

But then in some of the fight scenes, for instance I remember when he was fighting Guy and Naruto, he was about to touch Naruto but guy kicked him from behind so he didn't solidify and let himself pass through Naruto and let Guy's kick pass through him. Well at that time, why did he even need to transport his hand too? Couldn't he just transport the part of his body that was about to be kicked by Guy and then touch Naruto?

Why did he transport his fingers using the Kamui too? It was completely unnecessary and he could have taken down Naruto if he didn't do that..

Obito/ Tobi's kamui is his mangekyo sharingan ability unlocked presumably when he saw Rin die. It is a space-time ninjutsu that allows him to transfer objects into his own pocket dimension, including parts of his own body. While Obito is transfering anything between regular space-time and his own space-time his object not being transfered is vunerable

For Obito's ability

Here is the complete description on tobi

Tobi had displayed the ability to open up his top half entirely and use himself as a suit of sorts to encase someone in. Follow this link to get details.. wiki link

• oh sorry!! So Tobi is that masked hashirama clone? Oh...I meant Oboto..I edited the question!! – MartianCactus Mar 2 '17 at 7:54
• ok let me edit my answer – rahul Mar 2 '17 at 7:57
• The last bit is about Zetsu and not Tobi, I would omit that from the answer – Wouter Mar 2 '17 at 12:55

Kamui is an all or nothing technique. So while its reasonable to think that he could activate it for parts of his body only, he cant actually do that.

However, this eye can utilise a unique variation of teleportation akin to intangibility. When activated, any part of the user's body that overlaps with another object is seamlessly warped to Kamui's dimension, making it appear as though the user can phase through solid objects

When intangibility is active, he is completely intangible. His body is still in the real dimension until it overlaps, but absolutely any overlap will send the overlapped portion to the Kamui dimension. He can't attack or be attacked at all, and can't deactivate it while overlapped with something (or he can and it causes serious harm to him, we never see it happen). No known techniques work around this. He has never been harmed by someone in the real dimension while its active. its only weakness is that his overlapped real body sits in the Kamui dimension, and is vulnerable as normal to any attacks from the Kamui dimension, though if his head is also not there, he can't see it.