Space-Time ninjutsu of Tobi Why can't Tobi attack while he is dodging an attack using his Jikūkan Ninjutsu ?



  what Tobi actually does is warp away the part of his body that was being attacked to the other dimension.


During the entire anime it is portrayed that Tobi cannot attack while using Jikūkan Ninjutsu because his attacks will pass right through the target.   But while Kakashi was in Tobi's space-time dimension, he managed to punch him, thus indicating that Tobi wasn't intangible. 
Is there any explanation or any theory on this matter ?
Is there any explanation or any theory on this matter? Yes.

Kakashi discovers Obito's weakness, to which he explains how Obito's space-time justu works in the manga chapter 597, and utilizes it against him.


  Although Kakashi cannot use Kamui against Obito directly, he can use his own access to the other dimension to negate the advantages Obito makes use of: by teleporting attacks or allies to the other dimension at the same moment that Obito's body resides there for defence, he can be attacked successfully


Kakashi states that attacks "passing through" is Obito is incorrect - Obito sends his body into a different dimension; where he is completely tangible. 

Kakashi begins to understand how this justu works when he sends a kunai into the other dimension using his own Kumai ability at the same moment Obito teleports his head to dodge an attack from Naruto.