Core activation and neutral spine are terms that are often talked about. This video demonstrates simple ways to find the correct position.
Anyone who does heavy resistance training knows that it's important to be able to brace the core correctly. This is important for supporting the lower back when lifting, but it's also vital if you're to be able to lift more weight.
The ability to lift heavy things is governed by the Central Nervous System. It's clear imperative is to stop you causing yourself a catastrophic and life-threatening injury. Thus, if your Central Nervous System perceives a threat, for example the inability to support your lower back under load, it will shut down your ability to lift that weight.
First, you need to work out what core activation and a neutral spine position is and what it feels like. Then it's a process of mindfulness with regard to posture for a period of time. Finally, you teach yourself to hold that position in various positions when under load.