The format for the 20-rep squat goes like this:
You take a weight you can only comfortably squat for 10reps, then once you finish your 10th rep (which should be reasonably hard), you pause at the top, taking a few deep breaths, for as long as is necessary for you to pump out the next rep. Then do it again. By time you get to 15reps you should be begging for mercy, and your 20th rep should take a lot to get out.
Once you get past your 10th rep, *only focus on getting ONE more rep*. Then after that rep, focus on the next one. The idea is, you keep up this positive mindset until you make all the reps, or at least 15
Although a true widowmaker is a 20rep squat with a weight you'd use for 10 reps, it's still ok for beginners (like me) to do high rep squatting (i.e. 25 reps) with a lighter weight, but such that the full 25 reps are still a challenge. That's probably a gentle introduction to high rep squatting, and there's still some benefit to it
-shaun