How was Nagato able to re-awaken and control the Rinnegan?
The Rinnegan originally belonged to the Sage of the Six Paths. He then divided his power between his two sons, which formed the Senju and the Uchiha. For any human being to be able to activate Rinnegan, they need both the Uchiha and Senju DNA. Uchiha Madara, in the last days of his life, was able to activate it, as he already had the Eternal Mangekyo and Hashirama's cells (Senju DNA), making him similar to the Sage.
But the question is: how was Nagato able to re-activate it? Yes, it was Madara who had implanted his Rinnegan in him at a young age, but then, even though the eyes were implanted, Nagato neither had the Uchiha DNA nor the Senju DNA to be able to re-activate the Rinnegan. How was he able to do it?
And as a continuation to it, let's assume he was able to re-activate the already awakened Rinnegan (Madara awakened it originally), how was he able to use it so well and control it without his body taking a toll? He belonged to the Uzumaki clan who may have been the distant relatives of Senju, but that doesn't justify how he was able to control it so well as he did not have any trait of the SO6P, unlike Madara who could now be compared to the Sage (with both DNAs and the Rinnegan).
An example to highlight this is of Kakashi. He got a Sharingan from Obito. But since Kakashi wasn't an Uchiha, his body would take a huge toll on him whenever he used it and had to cover his Sharingan when not using to save Chakra. Though later he trained and excelled at it, he still was a non-Uchiha and thus, his body used to take the toll, though, only after excessive usage. Similarly, since Nagato has no traits of the Sage, his body shouldn't have been able to take the toll of Rinnegan activated full and controlling his 6 Pain paths, and all.
Can somebody please give a detailed explanation of how Nagato was able to awaken and control the Rinnegan?
Nagato did not need to awaken the Rinnegan. Madara already did that part for him (waiting for the Senju DNA to manifest and combine with the Uchiha DNA in order to awaken the Rinnegan). Since Nagato is an Uzumaki, which makes him a distant relative of the Senju, he can control the Rinnegan. All Nagato needed to do was activate the Rinnegan. (Senju relative + evolved Uchiha eye = Able to use Rinnegan)
The important part is Madara had the awakened Rinnegan and gave it to Nagato. Note that since Nagato received the Rinnegan and activated it, he never reverted back to normal eye form or Sharingan form. The best explanation for this is simply because Rinnegan does not cause eyestrain like the Sharingan and Byakugan. Rinnegan is the perfect eye form, and thus does not take a toll on Pain's body.
As for activation, we know that Nagato activated it upon the death of his parents. Keep in mind that these are Uchiha eyes and Uchiha has been known to have a stronger passion for love than Senju. Thus the activation of his eyes were out of love for his parents as he watched them die.
For control, the only requirements for Rinnegan is both Uchiha DNA and Senju DNA(the uzumaki clan were infact relatives of the Senju, sole holders of Sealing jutsu, to smoothen further ties and relations to the senju, and this point konoha, the 1st hokage married Uzumaki Mito, to further strengthen blood ties) which Nagato had. In general, mastering control comes with practice, just like with every other jutsu.
We know that Pain did not have full mastery of the Rinnegan as he didn't master the Outer Path. In order to use the Outer Path, Pain had to sacrifice his life when he revived everyone in Konoha.
"For control, the only requirements for Rinnegan is both Uchiha DNA and Senju DNA, which Nagato had." when was it shown that nagato had uchiha dna?
@krikara do you have any idea about this one? http://anime.stackexchange.com/questions/36135/why-could-kakashi-and-obito-move-while-naruto-sasuke-and-even-kaguya-couldnt