typing was ordinary...should have read...
the transverse abdominis...inner unit that stabilisers the spine which is the abs / core muscles main function
basically this means that all the crunches in the world won't make you better, stronger etc.
too many people are too concerned with loooks without function i.e bodybuilding
vacuums train the TA as stated above which are the ab muscles under the 6 pack muscles...these are the one's that make your middle area strong (think hitting a baseball) and their main function is to keep the spine straight twisting etc so basically over coming rotational forces...if they can't then back pain is the result leading to degenerative injuries leading to can't move when you're 50
if these muscles are activated / strong then they will hold your 6 pack muscles "up and in" (think tight) automatically giving you a slimmer appearence
couple that with proper training and good dieting and you'll have a 6 pack in no time
just a side point, no professional athlete's would do crunches, sit ups etc but their physiques are a product of there training (high intensity, clean diet etc)