Hello,
this reference is missing some explanation of the [imgxy] parameter. Here's what I've found in the mapserver-users archives:

When you don't click on the map itself, e.g. you click on the submit-type button of the form, then imgxy will be used as the center point for zooming.

If the map size is X and Y, imgxy will be "X/2-1 Y/2-1" (perhaps X/2 - 0.5 and Y/2 - 0.5 is more exactly).

For example, my img size is 220 300, then my form has this line:
<input type="hidden" name="imgxy" value="109 149">

( When you click on the map then img.x and img.y will be used )

http://mapserver.gis.umn.edu/data2/wilma/mapserver-users/0203/msg00409.html