Unrack explosively.

Drive your hips forward forcefully and press the floor away until the bar is completely free of the rack. Now pause just long enough for the bar and plates to stop moving.
GET YOUR HIPS THROUGH
GET YOUR HIPS THROUGH
GET YOUR HIPS THROUGH
Jeff Nippard
HIP THRUST UNRACK
HIP THRUST UNRACK
HIP THRUST UNRACK
Arch your [upper] back and push your hips back... and break the bar out of the racks by pushing your hips forward.
Unrack the bar by driving your shoulders up into the bar aggressively. A lot of people find that, especially with very heavy loads, unracking the bar aggressively makes the weight feel less intimidating than gingerly unracking the bar. Start with your hips a bit behind the bar, take a deep breath into your stomach, tense your back, and drive the bar up off the hooks by driving your hips under the bar and pushing the floor away from you.
Stronger By Science
I stand up like I am finishing a world record rep...  the reason why you're standing up so violently is because how this bar feels in your back from the moment that it leaves the rack is going to be very important for the outcome of this squat... Standing up explosively like you mean it is going to do a lot to make that bar feel lighter on your back.
Unracking is the way that you set the tone for the squat, so don't take it gently. Don't be timid about it. Be assertive with how you unrack the bar.
Szat Strength
As you’re getting ready to unrack the bar, think about pushing your upper back aggressively into the bar. Really try to feel the weight out, dominate it, before you ever take it out of the rack.
Robertson Training Systems
Next, stand up with the bar, and this part is key – hold it there for a second! Too often, you see people stand up and start stepping back all in one motion. This may be fine if you’re using what amounts to light weight for you, but as the weight gets heavier, you need to demonstrate more control. So stand up, and let the plates and your body settle for a minute. Feel it out, and then start to step back.
Robertson Training Systems
CONTROL. You gotta take your time so everything is tight, up, let the bar settle... A lot of times these guys go [up] and the bar's not settled so it's still moving, then they try to set up too fast and all of a sudden they're [shaking] because you get all that reverb going on the bar and your body hasn't absorbed any of it. So you want to let the body absorb it.
Super Training
Too often, people are... beginning their squat while other forces are still in motion, and the last thing that you want to do when you are trying to squat a heavy weight is deal with forces that you did not expect. So make your walkout methodical. Slow it down, take your time, build a ritual that you do the exact same thing with an empty bar on your back as you do with 900 pounds on your back... DO NOT RUSH IT. Once those plates quiet down, your body quiets down, and you have a brace... THEN it is time to initiate your descent.