Review on Enoch Ergonomic Mouse Wrist Rest: Comfortable Palm Wrist Support Pad for Office, Gaming, and Coding by Douglas Irwin

You can use the mouse tray both above and below your wrist (see photo). As someone who uses a combination of "fingertip" and "claw" grips on my mouse, I don't typically move my whole hand that much. So I used the mouse tray under my wrist, at hand. In this position, the mouse tray hardly moves, but gives my hand additional support. The only complaint is that the mouse pad came without any of the anti-slip pads. After informing the seller about this, he immediately offered to send me a replacement free of charge. However, since I don't need a mouse pad to slide my hand around, I decided against it as it doesn't affect my usage.