Pain allocates a portion of an individual's chakra to a living human sacrifice. This causes the sacrifice to become the individual's identical copy, complete with perfect imitations of the original's kekkei genkai (such as the Sharingan) and unique weapons (such as Samehada). The original controls their copy remotely, thus making the copy's movements and knowledge indistinguishable as well. But the copy is limited by how much of the original's chakra they are given, so particularly chakra-taxing jutsu cannot be used. Once the chakra from the original runs out, the sacrifice reverts to their own appearance and they die.
Pain uses the Shapeshifting Technique to make Yūra into a copy of Itachi Uchiha and Mukade into a copy of Kisame Hoshigaki. The copies are given 30% of their original's chakra and are used to delay Team Kakashi and Team Guy. Despite the copies supposedly being exactly the same as their original, Kakashi Hatake notices something different about the copy of Itachi, though he doesn't realise why until after the Shapeshifting Technique has ended.