By default a change to a content (page, block etc) is only mirrored if it is published (meaning a change of a non-published version wil not be mirrored). The approve workflow will in its final step publish the page which then triggers it to be mirrored, so it fits this scenario.

Another alternative if you want to have more control is to add some boolean property like "ShouldBeMirrored" on your content items. Then you can hook up an eventhandler to DataExporter.ContentExporting and in the event handler you can cancel mirroring of certain items if ShouldBeMirrored=false.

The value of ShouldBeMirrored could either be set by editors or you can have an eventhanlder hooked up to e.g. IContentEvents.PublishingContent and there have some logic to decide if ShouldBeMirrored should be set or not.