A suction formed by the concavity of the small of one's back as they lay on a hard plastic surface such as a tanning bed which simulates the action of a suction cup. This usually occurs in people who are slightly or greatly overweight due to the skin being looser. As one lays down they will find that air pushes out from a newly formed cavity created by the natural curve of the lumbar spine and, generally, the curve of a tanning bed. The "Love Handles", upper back and upper buttocks create a sealed ring against the plexiglass. This can create a weird or unpleasant sensation which can become embarrassing as an audible sound is created when the seal is broken when the subject moves. The sound is not dissimilar to flatulence.
I went tanning today and as I left I felt like everyone was staring because when I got of the the tanning bed my backuum seal broke and it sounded like a loud wet fart. The poor girl at the front desk probably brought toilet paper in to clean the bed!