tBTC, which lets people use Bitcoin to access Ethereum DeFi apps, uses a system of random signing groups to process transactions. Signers must bond an amount of ETH equal to 150% the value of the BTC being deposited, in order to disincentivize bad behavior. This 150% bond is forfeited in the event a signer does not perform their required function -- for example, sending a BTC redemption to an incorrect wallet address.
Members of the Keep Network can “delegate,” or stake, their ETH for a chance to be selected as signers, who verify and hold BTC deposited by users of the app.
tBTC requires a system with its own work token for Sybil resistance. This need is filled by the work token KEEP; since Keep handles custodianship for tBTC, those acting as signers normally need to stake both KEEP and ETH to perform their tasks. When tBTC signers perform their functions on the app properly, they receive fees in the form of KEEP tokens.