The "critical angle" is the angel on the pavilion at which a ray of light perpendicular to the table entering the stone, and hitting the pavilion facet will reflect and not pass through. Cutting below this angle will window, as the ray of light does not reflect back.
Now, if that angle for a certain stone is 38 degrees, and the pavilion was cut at 39, looking straight down into the pavilion, the stone will not window. However, if you tilt the stone 1.5 degrees, now you have effectively changed the angel of the pavilion in respect to your eye to 37.5 and you will see a tilt window. It would seem that the answer would be to cut the stone with a pavilion at maybe 45 degrees, and then you would have to tilt the stone very far to create the tilt window. The problem with this is that you will create extinction in the stone.
So what a clever cutter will do, is play with the crown of the stone. Bend the light on the crown before it gets to the pavilion. Crowns with smaller tables such as checker boards or other fancy cuts can virtually eliminate a tilt window, and extinction.