**I believe
Amaterasu is what you are reffering to**
Not to mention Tobi freely choped the same arm off when it became infected with Torune's insects.
Hmm...why not? Being observant is not just a trait of the Uchiha.
Here is the page tife is reffering to:
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It looks like we could have overlooked one minor detail. We all are too caught up in what Tobi intended to do when he touched Naruto, that we were too oblivious to the fact that Naruto just got a clean look straight at Tobi's MYSTERIOUS RIGHT ARM.
BUT It could just be that Naruto is shocked that Tobi is phasing through him because that is also what is happening.
That same arm is capable of blocking the executioner's blade. What type of tech is that? The only other similar tech we see in Earth Release users wielding techs that make their bodys firm as steel (Kakuzu as one example). The only other character we have evidence of moving freely through matter besides Tobi is Zetsu. I hate to lead to this conclusion but Tobi could be made of Zetsu parts or he has just mastered the same Earth Release tech.