I do this occasionaly... I guess from a bodybuilding perspective it would help you isolate the ab muscles in a pose.
When I do it, I breathe out as much as possible and sort of bend forward a little bit, then you make a sound and let a bit more air out, then straighten up and tense the abs.
After holding it for however long u want, let the air back in thru the nose. Don't let it in thru the mouth, as the air will come in too fast. Don't do lots of reps or you will get light-headed.
It was originally a yoga drill, I did a quick search, because I don't want to give advice on breathing drills without making sure it's correct. This article shows the yoga way of doing it
http://www.yogajournal.com/practice/871_1.cfm