Acceptance - S4-E4
Continuity mistake: The second time Hiro goes to the rooftop and tries to save Tadashi he has ink on his face from stopping the photocopier from working. However when we cut back to him from Tadashi the ink has vanished, then when we do get the next shot of him the ink is back on his face again.
Chosen answer: It's not been made 100% clear. It was originally assumed that he actually consumed part of the brain, but this has turned out not to be the case. Sylar has been shown all along to have an instinctive knowledge of how things work and how to fix them - this is his actual original power and the ability that made him an exceptional watchmaker and repairer. It appears that he uses a similar process when acquiring new powers - he exposes the brain of the target, allowing him to examine it and determine how their power functions. He can then apply that knowledge to his own brain, allowing him to replicate the ability in question.
Tailkinker ★