The object (the brown dot) is being rotated by 90° counterclockwise, around the centre spot, to its image, the the blue dot. Imagining lines meeting at this spot from each of the dots will help you visualize the rotation. Wherever you move the brown dot to, the blue image is always 90° counterclockwise from it.
Now click on 'Mode' and change the rotation angle to 180° by moving the red dot. Click on 'Mode' again, and move the brown dot. Notice this time that the blue image is always 180° away from it. It would be in the same place for a 180° clockwise rotation.
Now we want to show a 90° clockwise rotation. As explained on the information page, we will have to simulate this using a 270° counterclockwise rotation. Click on 'Mode' and do that now. A 270° counterclockwise rotation moves the brown dot around in a clockwise direction by 90°. Click on 'Mode' again to turn it off, and move the brown dot somewhere. Can you visualize the 90° clockwise rotation that results in the blue dot? Wherever you move the brown dot to, the blue image will be 90° away, in a clockwise direction.
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