One of the first things a new weightlifter should learn is how to set a rigid back in the start position. It's an old idea: turn the most flexible portion of the human skeleton, the spine, into a rigid conduit through which the upward energy created by back extension and leg drive is transferred to the bar as efficiently as possible. It is vital for a successful lift.
This post is not, however, aimed at newbies. There are plenty of coaches, articles, and videos out there to teach basic back position. ("Bracing" videos and advice seem to be everywhere on the internet right now.) What I want to do is encourage more experienced lifters to focus on it anew.
Over time some things thought to be trained-in habit can slip a little. You've been setting your back the same way for a while now, so you don't think about it much. You've built your setup into a consistent habit. But good habits can erode if you don't pay attention, and before you know it you are leaving kilos off the bar that could have been yours. I've seen experienced lifters let their upper backs get a little soft with warmup weights. The light weights still move quickly and precisely, so the imperfect back position goes unnoticed. But things go awry over 80% and the cause may seem baffling, even to your coach.
That's because your coach can get into a rut, too, their usually good eye slipping a little while paying attention to other things. She's watching for your most common technique flaws to crop up: you get pulled a little forward off the floor; you are impatient and don't wait for a good power position; you let the bar drift away from you as you move under. All of these can be caused by a back set that is a little bit soft. Then one day she thinks to remind you to arch and tighten their back more, engage your lats, and lock your shoulders down to your hips harder, and the jump in bar speed and precision is dramatic.
So when you set your back in the start, be extreme about it. ANY softness between the hips and bar will cost you bar speed. When I say be extreme, I mean EXTREME. I don't want you to get a "flat, neutral spine." Arch the shit out of your back, every pair of spinal erector muscles maxed out. Engage your lats when locking down your shoulders such that they threaten to cramp. Lift your chest and lock down your abs like your favorite crush is walking by. You can't hold this position long, so don't. Get tight and GO. You see world class lifters do that all the time. Even the lifters who seem to sit over the bar until the clock is down to single digits don't tighten up and sit there. They are relaxed until the short moment before they break the bar from the floor. Then they tighten like they are bracing to be hit by a truck and let it fly a half second later.
There is never a reason to half-set your back before a lift. Get in the habit of tightening it to your limit, and check in on that habit regularly. Then watch your bar jump!