Toucan's cross-chain messenger smart contract allows users to bridge carbon tokens between EVM chains: currently BCT, NCT, and TCO2s. Currently, the Hyperlane system is being utilized for EVM-based cross-chain bridging and communication. When tokens are sent from one blockchain to the other, they will be burned on the blockchain they are sent from, and then minted on the receiving chain. This means that the bridge will never hold tokens, and the amount of pool tokens in circulation on all chains will not exceed the underlying TCO2s.
Hyperlane is a generalized interchain messaging protocol that allows developers to send arbitrary data from one blockchain to another. Hyperlane provides an on-chain API to send and receive interchain messages. They provide permissionless interoperability. This solves the issue we have with permission interoperability solutions that new chains have to each register and be approved, which aligns with Toucans plan to build a multi-chain carbon infrastructure. Their documentation provides more detail on how Hyperlane works