Note that in this exercise the point is to avoid concentric muscle work, which in this case means actively bending your wrist upwards. Therefore, the other hand is used to bend the wrist and bring the dumbbell to the upper position.
Sit beside a table with your lower arm supported on the table. Hold a dumbbell with your palm facing down and your wrist over the edge of the table. Your wrist is bent upwards.
Slowly lower the dumbbell down. With the assistance of your other hand, return to the starting position by lifting the dumbbell and bending the wrist upwards.
