when doing a stomach vacuum are u trying to suck in your stomach as much as possible and contract them?

ive looked here but i gues not if it doesnt say to flex them right?

http://www.bodybuilding.com/fun/ms-fit5.htm

oh and do you hold ur breath the whole time?

i'm not sure about contracting your abs at the same time as sucking them in, but I know that you should take small, rapid breaths in and out as you do it so that you are neither holding your breath, nor filling your lungs with a lot of air.