Forget about your core and get EVERYTHING tight and learn how to do that without a belt.
Inhale using your diaphragm until it feels you have no more room and then inhale some more. Your belly should be very distended by this time. Hold that air. Squeeze down on it until you feel the pressure shooting up into your skull and out of your butthole. Squeeze the bar (for every lift this is always going to help), and then complete the lift. Do not exhale until the rep is complete, and repeat this process each rep.