We introduce a novel way for Alice to allow Bob to spend her UTXO/coins, without giving Bob her private key.
Previously, we have developed a way for Alice to sign arbitrary message in a transaction using her Bitcoin private key.
Bob adds his signature to the transaction and broadcasts it to spend the locked funds.
The delegate is undetermined in advance and can be chosen at will later, while a multisig has to know all delegates beforehand, which cannot be changed later on.
Basically, Alice signs Bob’s public key, Bob signs Charlie’s public key, … Line 13 validates the signing and thus delegation as before.