The circumstance where you find yourself in the position of needing to defacate but unable to locate a suitable venue for the proceedings. The resultant clenching of the sphincter in the hopes of preventing an incident of touching cloth
results in the feeling that the incipient turd is reversing back up into your lower intestines.
May be performed intentionally in order to preserve the waiting feces for later use. E.G in a cleveland steamer
Oh man, I hope we find a toilet soon. I'm shitting in reverse.
If I have shit in reverse again I'm going to end up spitting it out.