Before going any further, it's important to understand what a back off set is. Essentially, a back off set is a set(s) performed after your programmed working sets that will have altered intensity and reps to accommodate an increase in training at higher intensities.

What is top set and back off?

Essentially, the max-out, back-off protocol involves working up to a top low rep set in a particular exercise and then decreasing the weight and doing a number of back-off sets with the same exercise. The number of reps per set is usually increased during the back-off sets.

How heavy should back off sets be?

If you've ever got to the end of a workout and failed on the last set, but felt that your target muscles still weren't completely exhausted, back-off sets are the answer. Drop the weight that you're lifting by 40% for the final set and do as many reps as possible to take your muscle growth into new territory.