Using a wrist roller is an easy way to do wrist extensions with greater adjustability of resistance and complete range of motion. With a wrist roller or a roller setup on a barbell in a rack, rotate the hands toward yourself to roll the weight up, making sure you move the wrists through their complete range of motion. Reverse at a controlled speed to unroll.
 
It usually makes the most sense to combine extension with flexion by continuing to roll the weight back up in the opposite direction.
 
Purpose
The wrist roller strengthens flexion and extension of the wrist for general wrist health and stability and strength for supporting weight overhead.
 
Programming
2-3 sets of 2-3 times through is plenty. These can be done 1-2 days/week depending on what other wrist work is being done.