Turing complete smart contracts coming to Bitcoin in one way or another, wonder which implementation will be the winner of this race. Rollups, Spiderchains, Rootstock and now BitVM, all have different methods of bringing smart contracts to Bitcoin and all have their own trade offs

With BitVM it uses taproot to activates a contract and two parties can start exchanging off-chain data to trigger state changes in the circuit. 

Checkout the whitepaper here:

https://bitvm.org/bitvm.pdf