When I'm squatting heavy (of Course the term heavy is relative) about 70% of my attention is on my legs. A lot of people are so afraid of breaking their back that they squeeze their Backs way too tight. U just need to keep it straight and neutral not squeeze the crap out of your backs. This causes people to have a tendency of falling forward and then pulling the bar back in with their backs instead of standing with their legs. Just focus on legs and keep back straight.