Brace the upper arm on a preacher bench, glute ham bench, or incline bench holding a dumbbell with the palm up. Keeping the arm supported on the bench, curl the dumbbell up, and return to the bottom under control, avoiding hyperextension.
 
Purpose
The single-arm dumbbell preacher curl enforces strict curling to maximize bicep focus, and changes the resistance curve relative to a standing curl. Using dumbbell helps develop or maintain symmetry between the arms, and performing one arm at a time allows better focus on that arm, as well as allows more support options such as an incline bench.
 
Programming
Generally sets of 8-15 reps are appropriate.
 
Variations
Dumbbell preacher curls can be done with a pronated or neutral grip, or supinating during the curl.