Neutral spine.

Descend at a speed where you can maintain balance over your mid-foot and tightness throughout your trunk. For balance, your torso will naturally lean slightly forward, which is fine so long as you maintain that same angle throughout the rep. For tightness, ensure your back is flat from your tailbone through your neck.
Controlled Descent
Remember: the eccentric phase of muscle contraction (the lengthening phase) is very hypertrophic... you are here to tax the musculature. We want to control the way down... he controls the entire time on the way down and up - that means we're staying safe and our quads are active during the lengthening which means they're contracting eccentrically which is awesome for more muscle growth.
Renaissance Periodization
When most people descend quickly, they lose tightness, lose balance, and [so they're] not as strong as [they] would be... The only part [of your descent] that matters is staying tight, staying balanced. That's really all the matters throughout the entire lift.
I want you to go down as fast as you can go down in the squat... as fast as your technique can tolerate... as fast as you can maintain tightness [and] be consistent hitting your positions... We may want to be a little bit slower and more controlled in the top three quarters of the squat and then (if you're able to maintain tension doing this) to accelerate into the bottom of the squat and allow yourself to get some some rebound.. exploit the stretch reflex.
Juggernaut
Torso angle
DON’T DEADLIFT THE SQUAT
DON’T DEADLIFT THE SQUAT
As we descend, we want to make sure we set our torso angle at a maintainable position. We don't want to see a big shift or change in the torso angle throughout the squat - specifically as we hit the bottom and start coming back up. So on the descent, if we can get the appropriate angle, then we can keep it for the rest of the lift.
Calgary Barbell
As you squat down, it’s critical that you keep your chest up. This will help keep your lower back flat/neutral, and allow you to transfer maximal force into the bar.
Robertson Training Systems
There should be no drastic changes in your back angle during the ascent portion of the squat. If you descend with a vertical torso, stand up with that same vertical torso. If you descend with a more horizontal torso, stand up with that same horizontal torso.
Alan Thrall
This brace should not be an arch. We’ve spent all this time trying to get into an ideal spinal position, so stay in a neutral spine position! ...make sure the head and neck are neutral as well. Often this requires you to pull the head/neck back a bit, to get them in-line with the rest of the spine.
Robertson Training Systems
Being super bent over in the lower back doesn't do anything good for injury risk but... it reduces your ability to produce force through the quads. The back becomes a limiting factor... you should at least have your spine relatively straight, brace nice and tight, keep your chest not necessarily super up but don't let it cave forward... At the bottom of a deep squat, gravity and body mechanics want you to fold over on yourself - just don't do it. Resist by maintaining upright posture... [with the] back either neutral or in a slight lordosis.
Renaissance Periodization
I don't ever think about trying to arch my lower back. I think about trying to drive my core out and stabilize, and if I do that correctly I stay straight.
Super Training
Constant head, chest, and torso position! [When I] pick my chest up out of the bottom, this shifts my balance. The barbell is no longer over the middle of my foot and it takes the tension off my hips, making it difficult to grind through heavy reps. Maintaining the correct torso position and knee position will allow you to stay in your hips during the entire squat... Now I'm looking down and out in front. I'm burning a hole in the ground with my eyes... Keeping my nipples down towards the floor helps me maintain the correct chest and torso position.
Alan Thrall
Keep your chest up, your upper-back tight and the bar over your mid-foot. Don’t let your hips rise faster than your chest or you’ll lean forward and end in a dangerous position... Maintain the back angle you had at the bottom.
StrongLifts
Flat neck
When you [look up] you are placing your spine in a position where it can simply support less weight... like driving your car with the emergency brake on... it is very very important that from the moment that you unrack the bar until you put the bar back in the rack that you keep your chin nice and neutral and it doesn't change position. Just fixing that alone will often solve many other structural issues in the squat as well as up your bar speed which is always an excellent thing when it comes to moving heavy weights.
NEUTRAL SPINE
NEUTRAL SPINE
Juggernaut
You want a neutral head position... looking up is not a good idea. That's going to put a curvature into your spine and the curvature into your bar path... Typically I look slightly down.
Super Training
You're gonna have this tendency to hyperextend with your neck... it turns on all these muscles on the back side of your body... and drive you into an anterior pelvic tilt... It's going to shut off your glutes and your hamstrings... So [instead] we want neutral neck, eyes up... you can see he's neutral through his lower back and his hips. This is gonna allow him to use his glutes and his hamstrings when coming off the floor.
Robertson Training Systems
... you want to keep your chin as neutral as possible, which basically means that you're just trying to stare at a point that's about 10 to 12 feet in front of you... and stay there...your chin stays straight down [and] neutral.