There are two buttons for this option.
Once the button has been selected, the sequence of events to invoke the flip is :
< ButtonPress-1 > - in one of the points on the bounding-box.
The flip occurs using the x/y -coordinate of the selected point as the centre.
So if the Flip About-x button is pressed, clicking in points 1, 4 or 6 will result in the flipped shape apperently flipping to the left. Similarly points 3, 5 and 8 will result in the shape apperently flipping to the right. Points 2 and 7 result in the shape being flipped within the box.
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Gudge.