How does the mouse tracking test work?
Press Start and move your pointer onto the target; after a moment it starts to glide around the box for 20 seconds, turning and changing speed smoothly. Keep your pointer on it. The target turns green while you are on it. On a phone or tablet, keep your finger on it instead. The route is different every time.
How is your result calculated?
On every frame the screen draws, the distance between your pointer and the target's centre is recorded. Your result is the share of frames in which your pointer was on the target. Frames where your pointer was off the box, or your finger was lifted, count as off target. You also see your average distance, in multiples of the target's radius.
What makes tracking hard?
Your hand always reacts a little late. To stay on a moving target you have to predict where it is going rather than chase where it is, and every change of direction breaks the prediction for a moment. That is why the target here moves smoothly: it can be predicted, but only by following it closely.
Is this a test of my mouse?
No. This tests how well you follow a moving target, not your hardware. It doesn't measure a mouse's polling rate, sensor accuracy or lag. A very slow or erratic mouse can lower your score, but so can many other things, so the result says little about the device itself.
For a test of aiming at targets that stay still, try the Mouse Accuracy Test; for keeping track of several moving things with your eyes, the Multiple Object Tracking test.