Ali Atiia described it as such:
An optimistic rollup chain is really just (1) a smart contract on some layer-1 chain like Ethereum mainnet and (2) block producer(s) that listens for transactions on a dedicated P2P network, batch them up into one big string of data, and post it to layer-1 along with a signature attesting to their validity. This attestation can and will be used against them in the court of the ORU smart contract if they commit fraud.
The contract accepts transaction directly from users in case the Operator is censoring. That allows a user to exit the ORU safely with their funds if the Operator goes rogue or unresponsive. And importantly, the contract can verify fraud claims and slash the operator’s bond if indeed there was fraud.
It must be the case that the data attached to an ORU transaction is all a whistle-blower needs to detect and report fraud. Layer-1 full nodes never execute what’s inside the attached data (unless someone reports fraud) and that is where the scalability gain comes from. To layer-1, that data is just a binary blob to be stored forever.