The enforcement mechanism is mostly paranoia. The ethical codes for attorneys requires you to rat out your colleagues at the threat of getting in trouble yourself if you fail to do so. Documents are managed in review databases with time stamps and user tracking. Opposing counsel will go through your productions and might spot inconsistencies. Opposing counsel will also spend hours grilling witnesses, looking for inconsistencies and references to documents that haven't been produced. Opposing counsel also generally has a good idea of what kind of documents should exist, given that corporate transactions are generally pretty stylized.

At the most basic level, the system is built on trusting lawyers to act dutifully as officers of the court. Corporate law firms care very much about their brand for trustworthiness, because at the end of the day their business depends on that brand. Nobody wants to be like Arthur Andersen, which went from a $9 billion company to nothing because people stopped trusting their brand (for the actions of a relatively small group of partners).