RSK is a Bitcoin Sidechain — a separate blockchain that carries a "separate token" but is able to exchange tokens back and forth with the main Bitcoin blockchain. Users can move bitcoin to the RSK chain, make use of the converted bitcoin, and then eventually move their tokens back, as bitcoin, to the main blockchain by a 2waypeg(aka making decentralized bitcoin backed tokens) . Summing this paragraph off-rsk.co uses a decentralized process to create bitcoin backed tokens
Here how they peg bitcoin to smart bitcoin. Remember 1 smart bitcoin=1 bitcoin

No hardfork needed it, it merged mine with same chain.