This is a relatively new concept, where control on unlocking the locked bitcoins (in main chain) is given to the miners who can vote when to unlock them. This is in contrast to sidechains, where consensus is validated though simple payment verification mechanism in order to transfer the coins back to the main chain.

Rootstock is a smart contract platform which has a two-way peg into bitcoin blockchain. The core idea is to increase the scalability and performance of the bitcoin system and enable it to work with smart contracts. Rootstock runs a Turing complete deterministic virtual machine called Rootstock Virtual Machine (RVM). It is also compatible with the EVM and allows solidity-compiled contracts to run on Rootstock. Smart contracts can also run under the time-tested security of bitcoin blockchain. The Rootstock blockchain works by merge mining with bitcoins. This allows Rootstock blockchain to achieve the same security level as Bitcoin. This is especially true for preventing double spends and achieving settlement finality. It allows scalability, up to 400 transactions per second due to faster block times and other design considerations.

RSK has released the main network called Bamboo, RSK MainNet which is a beta currently.