John,
You explained it pretty well yourself. The bird's are released from a tower 20' to 30' above the ground. The shooter's stand in a circle surrounding the tower. Each man is just out of shotgun reach of the tower about 30 - 40 yards away. The birds are released one at a time and the shooters move position's every so many birds. This gives the shooters a chance at all of the positions.
If the wind is blowing pretty good the birds will all fly in the same direction with the wind pushing them. If you are on the other side of the circle you would get a chance when you rotated to that side of the field.
It's a lot of fun for an organized shoot.