The wrist extensors are the muscles on the outside of the forearm that tense when you bring your wrist back. They do get tight quite easily, and left untreated can develop into Tennis Elbow. This exercise can be a good preventative one for people who use these muscles a lot- like tennis players. However it can be a bit strong for Tennis Elbow in the early stages, so see your osteopath before you start this exercise if that applies to you.
Start with a small spiky ball (mine is from this set), and just use your hand to roll it around the wrist extensors. You’ll most likely find one spot where the pressure gives you “good pain”, so focus around that. You can follow the muscles up and down, or work across. Good pain is alright, but don’t work so hard or long that it feels more tender than when you started!
You may find that using heat or ice before using the ball helps with pain, or makes it more effective. Ask your osteopath which one is more appropriate for you.