Glutes!

As your drive your hips through, squeeze your ass cheeks hard.
Squeeze your cheeks
SQUEEZE THE GLUTES
SQUEEZE THE GLUTES
Alan Thrall
Too often, I see someone standing up and locking their knees out, but failing to lock out their hips.... What I want you to do instead is squeeze your glutes together and get your hips through. Make love to the bar... Squeeze them booty cheeks and violently fire your hips forward.
Alan Thrall
One sign: if you're not engaging your glutes enough, you're probably going to know about it because you're not even really feeling them fire at all.
Omar Isuf
The cue that I like to tell someone to finish tall, finish with their hips through... The idea is you want them to finish tall - not over-extended or where their butt's sticking out.
T-Nation
One of the biggest changes I made in my deadlift was glute activation and the biggest, best thing ever for me was was this in particular: really really bring them through. When I say bring them through, you should be able to just hit your glutes - rock hard. Really really over-exaggeratedly squeeze them. Nice and rock hard there.
Omar Isuf
Pretend you are in prison and you were trying to send the message that you are not open for business.
Alan Thrall
Push away, squeeze your ass. Because that's gonna meet the hips to the bar.
Szat Strength
As soon as that bar gets to the top of your kneecaps, you want to engage your butt. You want to start flexing and trying to drive your hips forward. That is going to help that bar go vertical... You'll be shocked at how it will help solve some of your lockout problems.
Neversate