21′s consist of 3 parts:  

 Set Up: Begin with Dumbbells by your side

Part 1: Curl Dumbbells halfway up until your forearms are parallel with the ground. Lower to starting position. Repeat 7 times.
Part 2: Curl the Dumbbells to the top, but lower only halfway, to the midpoint where forearms are parallel to the ground. Repeat 7 times.

Part 3: Perform the full range of motion for a normal bicep curl. Repeat 7 times.