Countdown sets are performed at the end of your workout to maximize workout density and push yourself past your limits.

Here's an example using a dumbbell row:

1. Pick a weight you can row for 12-15 reps.
2. Do 10 reps with your right arm, 10 reps with your left arm.
3. Now do 9 reps with your right arm, 9 with your left.
4. Continue this way, back and forth without rest, all the way to 1 rep.

In a matter of minutes, you'll have added over 50 reps of skin-splitting training volume with a moderately heavy weight.

You can apply the 10-1 method to any compound exercise, but start light and only do one set. If you're doing countdown sets with a bilateral (both arms or legs) movement, rest 15 to 30 seconds between sets.