Nucs are usually on the move. They are set in one location and probably taken to a different yard for a new emergin queen to mate with drones (mating yard). They are also driven around carrying queens for replacing failing queens in distant yards.
The beekeeper may sometimes decide that no queen should be out of her nuc or for transport the beekeeper may decide that no bee should be allowed out (while keeping ventilation or not).
This Nuc Entrance Gate can be added with a screw to the front of a nuc that has a single hole for bee entrance. The screw needs to be tight enough for keeping the gate in place but loose enough for allowing the gate to be turned around.
The gate has 4 positions:
The gate has a diameter of 5" and the nuc entrance hole should be smaller than 2" wide.