This is getting into implementation details, so pay close attention to the parts which seem contradictory. Conceptually anyone who wants sidecoins and has bitcoins or vice versa can simply move value from one chain to the other, in essence destroying bitcoins and printing sidecoins albeit in a reversible process. However, this takes a considerable amount of time to clear -- about 3 days -- and blockchain space which requires fees. So in practice we fully expect that someone wanting to trade bitcoins for sidecoins will use a distributed marketplace and atomic cross-chain swaps in order to buy one coin with another, by finding a buyer at the other side. Use of the pegging mechanism will be limited to market makers who arbitrage when the price starts to deviate from the peg. Make sense?