Reflecting across the line (y=x) means the point is flipped over the diagonal line where the coordinates are equal. The key effect is simple:
The line (y=x) is exactly the set of points where the two coordinates match. Reflecting across it exchanges horizontal and vertical position while keeping the same distance from the line.
A quick check is to make sure the original point and its image have reversed coordinates. If you swap them back, you should recover the original point. Also, if a point lies on (y=x), it stays in the same place after reflection.
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