I push my abs out as I breathe in because I inhale by expanding my diaphragm. When I've got as big a bellyful of air as I can then I squeeze my midsection as hard as possible, but once I squeeze I don't push against the belt, just squeeze down on the air. Lats get tight too, for deadlift. Then I start moving.
I know, there's a heap of information out there and it can be confusing. You'll eventually figure out the good from the bad. I think the varying views come from some some people basing their take on just their own experiences. What works for one won't necessarily work for another.