the phenomenon whereby the anus possesses it's own brain and makes it's own decisions.
brain + anal = brainal
You are walking home and the urge to shit hits you, the brain in your head tells your anus to tighten and you continue walking. You are doing very well until close to home when all of a sudden you're scrambling for your keys and fumbling to open the door, running to the toilet whilst undoing your pants and just making it in time (or sometimes not making it).
This situation occurs because the brain in your anus overrides the brain in your head. Despite your best efforts to clinch and tighten, your brainal can sense when a toilet is near and immediately starts to open the brown gates.