To let air be sucked into the anus to be blown back out in the form of a fart.
"Dude, I was laying awkwardly in bed on my stomache, when I relaxed my sphincter and let air flow in. Then I let the air back out as a fart. I call it seeping!"
To lay on any horizontal surface stomach down with your head laying on the surface, on your knees, and your bottom hunched out in the air, whilst you proceed to relax the anal sphincter as to let air flow into the anus. You will then have obtained the feeling of a need to flatulate, and thusly you would expel air from your rectum as it "seeps" up.
I couldn't sleep last night, and after rolling around in several positions, I finally rolled into this one position where air somehow slipped into my anus and made me feel as if I had to pass gas. I call it "seeping". I repeated the process all night long. I never got to sleep.