This feature will allow you to "undo" any changeSet you "regret". Let's say in a previous migration you executed a changeSet that now realises should never be added. In this case is where UNDO operations come handy. You tell Mongock to execute that operation to "repair" the actions of the undesired changeSet. You may wonder , why not add a standard changeSet that does the "undo" job. While this would work, it will force Mongock to always run the undesired changeSet. So it could happen that you provide an UNDO operation for a changeSet you don't want to be executed, but it will, to then later, "fixed". When providing an UNDO operation associated with a changeSet, Mongock will check if the changeSet has been already executed. If that is the case, it will run the UNDO operation, otherwise neither the changeSet nor the UNDO will be ever executed.