Drivechain allows BTC to travel back-and-forth to other software applications (“sidechains”). Thus, BTC-owners can opt-in to new features. Bitcoin Core nodes don't care what a sidechain does. Sidechain-deposits are instant and zero-trust. Withdrawals are hashrate-controlled and arbitrary, but rate-limited (1 per 92 days) and audit-able (compressed to 32 bytes). Sidechain users can sell their default risk using atomic swaps – also instant and zero-trust.