you will definitely be taking more than a breath with 20 reppers. Espeacially as you get to the last 4-6 reps.
You can tailor 20 reppers to however you see fit. Most use them as they are, no box.
You can do them with 1-4 second pauses at the bottom, you can do tempo 20 reppers as Well.
Some have done 20 reppers for “out of the hole” work, some use them for “descent” work.
It’s really up to you how you wish to go about it. Most of the cues should be focused on your form, however things can get iffy when you’re being too specific, because your form more than likely will get shifty regardless to whatever extent, be it slight or a bit more. Which is another reason why you shouldn’t have a very heavy percentage of weight across your back when doing 20 reppers. Challenging? By all means yes. Enough to keep general form still in tact.
Just keep in mind cues that suit your particular squat setup, and if you can manage, to re-adjust yourself as best as you can if needed, and keep yourself tight as much as you can.
Most of your focus should be centered around driving out of the hole, and making sure the glutes, hams, calves, and quads are all doing their part, as well as bracing, keeping a neutral and tight back and neck, and watching your weight distribution in your feet, since it’s easy to overload an area for sake of trying to take the tension off of another, espeacially as you become more and more fatigued.
Hope this helps in some way.