Perform multiple sets of 1 rep using between 85 and 90 per cent of your current 1RM (one rep max, or the most weight you can lift one time using good form and without experiencing pain) and 60 second rests between each set. Perform a maximum of 15 sets.

If, on any given workout, you manage to perform 15×1 in good form using 60 second rests, the next time you perform that same workout, increase the load by 5 percent or 5 pounds (whichever is smaller) and repeat. If, on the other hand, you do NOT manage to complete 15 sets, the next time you perform that same workout, simply try to work your way closer to 15 sets. In all cases, on every fourth week you should reduce your total volume by one-half. By way of example, if you performed 15×1 on squats using 275 pounds on week three, then perform 7-8×1 in the same 15 minute PR Zone (time frame) on week four.

This explanation was taken from the 21 Day Itch article by Fitness Genius Charles Staley posted on T-nation.com.