[Ensam]

You should be engaging your lats throughout the whole exercise, from the moment you touch the bar to the moment you set it back in the rack. Think about how your back is at the top of a pull up, that's what you want to feel like through the squat. Another way to think about it is to try and break the bar across your back. Fully engaging your lats will stabilize your spine and make the weight feel much lighter. Same goes for deadlift. I saw my biggest gain in lat size during a period of time where I was only doing squats and deadlifts - no targeted upper body work at all.