A reed switch normally goes ON when a moving magnet is near.
Adding a second non-moving magnet to balance the field will make it OPEN when the moving magnet is in the right spot.
Step 1: The Norths and The Souths of it
The magnet with the north pole at the top is adjusted until the switch turns OFF with the other moving magnet in it's 'special' position.
When the moving magnet moves either closer or farther, the switch turns ON.