It can be implemented either way - it's up to the sidechain. For example, a simple sidechain might lock its exchange rate at 1=1 to the bitcoin network (if the

It can be implemented either way - it's up to the sidechain. For example, a simple sidechain might lock its exchange rate at 1=1 to the bitcoin network (if the creator just wanted to test out protocol improvements, such as faster block times). On the other hand, someone could create something totally different, like a lotto coin, where each week one of the holders is allowed to "withdraw" all bitcoin in the network back to the bitcoin blockchain. All that matters is that all the miners/participants in the sidechain agree what the rules are for importing and exporting bitcoin. If the sidechain miners allow the withdrawal to go ahead (and it reaches enough confirmations to mitigate a double spend attack), then the bitcoin blockchain allows the original coins to be unlocked.