The wrist curl is a training exercise for only develop the muscles of the wrist flexors of the forearm. It is therefore an isolation exercise. Ideally, it should be done in combination with the "reverse wrist loop" (also called wrist extension) to ensure equal development of wrist flexors and wrist extensor muscles.
One Arm Palm Up Wrist Curl :
➋ Lifting a dumbbell in one hand and rising slowly lower back off once a brief pause.
➌ Alternate hands once a group. Keep your arms still long.
Palms Up Wrist Curl :
➋ Lift the dumbbells lifting solely your hands and slowly lower them back off once a brief pause.
➌ Keep your arms still long.