Start with a weight you can do for say 12 reps. Do 12, then increase your weight so you can only do 8 or so, then 6.
Example
Bench press - 135X12, 160X8, 180X6
Basically, as you increase your weight, you lower the amount of reps. I think the argument for it is that it gets you up to your max weight for the day using small steps, not a big jump from the warm-up to your max weight for whatever many sets or reps. The argument against using it is definitely that you use a lot of your energy using sub-optimal weight, so you have less energy to use when your weight gets higher.
ND