MPC

Multi-party computation splits a key into shares held by separate parties, who jointly produce a valid signature without any of them ever holding the whole key. There is no moment in the lifecycle, including generation and signing, at which the complete key exists in one place. That is the property that distinguishes it from splitting a key up and reassembling it later.

Against multisig the trade-offs are concrete. MPC produces an ordinary single signature, so it costs the same as any transaction, works on chains with no native multisig support, and does not reveal the quorum structure on-chain. Multisig enforces its policy in a smart contract that anyone can audit and that no vendor can change. MPC moves that policy off-chain into the implementation, which means the security depends on a protocol most institutions cannot independently verify.

Proactive refresh is the feature worth asking about. Shares can be re-randomized on a schedule so that shares captured at different times cannot be combined, which turns a slow multi-party compromise into a race the attacker has to win inside one refresh window. Ask any custody vendor how often refresh runs and what happens to a share holder who is offline when it does.