I've heard buckos say that you engage your core in the same way you'd brace yourself if somebody were about to give ya a thump in the stomach.
People naturally engage their core when they're laughing apparently as well!
Though it's important to make a distinction between sucking in your stomach, and engaging your core: the latter is more like the notion of bringing your navel closer to the spine. You should be able to breathe properly whilst engaging your core.
I hope this helps Lego! If you're uncertain, there's endless amounts of videos and articles on it too if you're still a bit confused.