So right now the best "two way peg" for bitcoin scaling layers is multisig and federated solutions. Drivechain is two bips proposed in 2017 that allows for a new type of address called a hashrate escrow to hold the "drivechain" L1 bitcoin. Users send bitcoin to one of these addresses and recieve an equal amount on the drivechain. Drivechains would be blind merge mined so basically they piggy back on btc PoW and pay miners the sidechain txn fees to anchor the sidechain block header into BTC mainchain. To get funds out of the drivechain it takes 3-6 months of a slow very public process allowing for any public intervention should a majority of hashrate be compromised and trying to steal the DC funds.

Its a way to make trust minimized bitcoin sidehains for scaling and experimentation. There are already a handful of merge mined sidechains you can see in the coinbase op_return