I'll try and give it a shot. The antenna pattern is shaped like a donut, like the picture at:
http://www.fmx.dk/projects/wing_cam/119-1935_IMG.JPG

In this picture, imagine the ferrite loopstick sticking horizontally through the donut, with nulls at either end. The null zone is actually conical, meaning that you have to point the null properly in all three directions, including azimuth (elevation).

Now imagine a local pest that is coming in at, say, 45 degrees above the horizon. If you left your loopstick perfectly level, as in the picture above, signals coming from 45 degrees up will still be heard quite nicely. However, if you point it upward 45 degrees, the maximum null will also be 45 degrees up, matching the elevation of the pest's signal, and therefore the null is achieved.

For signals from far away, the angle of arrival is perhaps a few degrees at most above the horizon, and nulls are generally achieved with the loopstick essentially level, since the null of the loopstick is pointing more or less at the horizon

Hope this helps - Kevin S
Bainbridge Island, WA