Note the holes for the photocells to shoot through on yellow Festina donut. UCI rules now allow the rider to start within a "window" The true time is taken when the riders wheel passes through the photocell beam. Note the height is set so that the leading edge of the wheel trips the cell.
http://www.cyclingnews.com/photos.php?id=photos/2004/sep04/gpdesnations04/image031e
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