Blown SC
I've caught more **** over this than any other aspect of training. But the truth is that every big squatter I know has learned how to use his abdominals while squatting. You must learn how to breathe into your belly. You want to pull as much air as you can into your belly, then flex and force your abdominals out.
Walk over to a mirror. Take a look at your shoulders and take a deep breath. Did they rise? If they did, then you're pulling all the air into your chest, not your belly. You need to learn how to breath into your belly. This is how we teach everyone to squat. For the squat, we advise the use of a weight belt worn one notch loose. This is to teach you to pull air into your belly then push out into the belt. The belt acts as a great training aid to push against.