the transverse abdominus lies deep to the rectus abdominus (six pack). the fibers of that muscle (TVA) run, basically, around your body like a corset, so when you contract said muscle it shortens. basically you tighten the belt. over time you develop some tone in that muscle similar to what was mentioned with overdeveloping the chest (hunching over). that tension causes the waistline to draw in. having control over your TVA is not only beneficial to a persons posture and core stability, but also for bodybuilding purposes in posing.