You have to sit pretty strangely, like leaned back with your arm fully against your side so the weight of your forearm pushes into you rather than pulling on your shoulder. It does trade a LOT of hand strain for a slight bit of shoulder strain though.
I used a vertical mouse for a while and found that it puts more strain on my shoulder and ended up going back. Your mileage may vary
You have to sit pretty strangely, like leaned back with your arm fully against your side so the weight of your forearm pushes into you rather than pulling on your shoulder. It does trade a LOT of hand strain for a slight bit of shoulder strain though.