By all rights, the arm should have disappeared back into the ether when Archer faded away. It was testament to Kirei's skills that the man had been able to graft the limb onto Shirou's Magic Circuits, allowing it to remain in this world by using his own Prana as a source of power.
Shirou honestly didn't know what Archer had been thinking when he'd proposed the operation, but as it stood the arm, or Artificial Phantasm as he'd come to regard it, was both his Ultimate Weapon and Instrument of his Destruction in one.
On the one hand, it passively fed him Archer's memories, history and, more importantly, his Battle Experience and knowledge of Projection Magecraft. On the other, putting aside their shared origins, there was a stark difference between the Magus Emiya Shirou and the Counter Guardian summoned as the Archer of the 5th Heaven's Feel.
Archer had regretted sticking to his ideals after being betrayed by them and in turn being forced to betray them after becoming a Counter Guardian, whereas Shirou willingly cast aside those ideals to protect those important to him, only for the world to spit upon his actions as if punishing him for daring to be selfish after so long.
In the end, they were two entirely different people, and as a result their inner worlds, the very basis of their Magecraft, clashed and tore one another apart in a bid for dominance, as no matter how similar they were in life, a Human Being could not compare to a Heroic Spirit, to the point that even tapping into the abilities of the Artificial Phantasm for a second ran the risk of Shirou being swallowed whole by the 'Flow' of Archer's power.
It was for this reason that Kotomine had given him the Shroud of Martin, a conceptual weapon that allowed them to partially 'seal off' the remains of Archer's Circuits within the Artificial Phantasm. While far from a perfect solution, as even without employing Magecraft Prana continuously flowed through the body, forcing Shirou to subconsciously erect a barrier between his Circuit's and Archers to keep them from activating, it did lessen the strain on him to the point he could live an ordinary life.
On the upside, even with the effect reduced, the passive boost to Shirou's abilities allowed him to easily surpass those of a normal human. On the downside, the resulting initial feedback from moving the arm for an extended period of time with the circuit active was akin to being cut and stabbed by his own body.
During his initial controlled experiment, wherein he partially undid the bindings to test his limits, the sheer agony had been so intense he honestly couldn't recall what he'd been doing when he came to, save that he'd narrowly escaped death atop a Hill of Swords. It had been the first time in ten years he'd truly feared for his life as he had during the night he'd wandered through the Fuyuki fire. Even being hunted down by Lancer, staring down Berserker or struggling against Gilgamesh hadn't scared him to the point he actually feared for his life.
There had even been times he'd considered cutting the damn thing off, but without it he wouldn't have been able to stand beside Rin and Ilya when they'd gone to confront Zouken and Sakura. That, and a part of him had wanted to live up to the idea Kotomine proposed: That if he honed his skills to the point they matched Archer's, he'd be able to make the Arm his own without having to rely on the Shroud to seal it.
As someone whose life had been built upon borrowed, flawed ideals, the idea of moving beyond relying on Archer's skills, to take what the Counter Guardian had developed and make it his own, was something Shirou found wholly appealing. While there hadn't truly been time to experiment prior to setting out to destroy the grail, since awakening and regaining his strength, the Faker had used his free time to carefully build upon what little he'd garnered from Archer's Knowledge on Projection Magecraft.
Despite his growth, however, he still couldn't trace A-Rank Noble Phantasms without relying on the power of the Artificial Phantasm, nor could he 'Break' a Phaantasm without running the risk of it exploding in his face. Still, the fact he was merely bedridden after using the Crane Wing was proof enough that he was far closer to mastering the Arm than he ever would have been back home, a fact he attributed partially to the over-abundance of Prana and the lack of interference from Gaia.
Thanks to these, he was able to increase the amount of time he was able to interact with the limb, essentially allowing him to treat Archer's memories as 'Cheat Notes', which combined with the reduced costs for Tracing made it much easier to progress with his chosen Magecraft than before. That being said, while he could Trace anything up to Rank-B, the only weapons he felt comfortable weilding in battle were Kanshou and Bakuya, the married twin blades that, while Noble Phantasms, lacked a true 'owner', allowing Archer, and Shirou by proxy, to incorporate them into their personal style with relative ease.
'At the very least I know using Crane Wing won't kill me.' he mused, smirking wryly as he rolled out of bed, grimacing at the feel of the sweat-stained sheets, making a mental note to change them as he made for the shower, only to pause at the sight of the sunset outside.
'Damn…missed an entire day…' he muttered, irritation warring with resignation as he grabbed a towel and made for the bathroom, hoping that Azazel didn't forget to call in to explain his absence, as he didn't feel like getting into an argument with Sona, who would no doubt blame it on his part-time job 'Hope I didn't miss anything at school…'
Kuoh Academy Occult Research Club Room...