Keith Fielder, an extension agent in Georgia, gave a talk last month at our bee club. He uses only push-in cages when he introduces a queen. You make it yourself of #8 hardware cloth - it's about a 4" X 4" square. You fold sides down to allow the cage to be about 1/2 " tall and use the cut wires on the folded edges to push the cage into the comb. He dumps the queen out of the cage in which she arrived onto the screened wire and pushes it into the comb, caging her in the now enlarged space for her to begin in the hive. The queen then has an area in which to start laying and she is protected by the push-in cage while the hive gets used to her.
That was my understanding from his talk. Hope that makes sense.
Here's a good description: http://members.aol.com/queenb95/QnIntroInstr.html
And here is MBush's description of one:
"Push In Cage
This is the most reliable release for a laying queen. The concept of this is to give the queen some newly emerged attendants, who will accept her since they have never had any other queen, some food and a place to lay. Once she is a laying queen with attendants the hive will usually accept her without protest.
Making a Push In Cage
Most people make these about 4 inches square (10 cm). I prefer to make them bigger. The larger they are the easier it is to get some honey (so she doesn't starve) some open cells (so she has a place to lay) and some emerging brood (so she has attendants). I like mine about 5 by 10 inches (12.5cm by 25cm). Cut some #8 hardware cloth (8 wires to an inch or 1/8" wire cloth) 6 1/2" by 11 1/2" (about 16cm by 29cm). Pull off the first three wires all the way around leaving 3/8" wires sticking out with no cross wires. This is to push into the comb so that the bees can't get under easily. Now come in 3/4" from the corners (three more wires) and make a cut 3/4" in (3 more wires) on all four corners. It really doesn't matter from which direction, but you're going to fold it around the corner. Fold the 3/4" edge over. A board or the sharp edge of a table is helpful in doing this. Fold the 3/4" corners over. You now have a box with no bottom that is 3/4 inch tall and 5 by 10 inches.
Using a Push in Cage
Find a comb with emerging brood. This comb is bees who are fuzzy and struggling to get out of a cell they have just chewed open. A bee with its head sticking out of a cell is emerging brood. A bee with it's behind sticking out of a cell is a nurse bee feeding a larvae or a house bee cleaning a cell. Shake (if the comb is strong enough) or brush all of the bees off of the comb. Release the queen on one side of the comb where there is emerging brood and some open honey. Put the cage over her so that it has both honey and emerging brood in it. Some open cells are nice too. Push the cage into the comb. It should stick up about 3/8" above the comb to make room for the queen to move around. Make room in the hive for this frame plus the 3/8". Some will have enough space and some will have to have a frame removed, but you need to have the frame with the push in cage and then 3/8" space between the cage and the comb on the next frame so that bees have access to the cage to meet the queen and feed them if they like. Come back in four days and release the queen by removing the cage."
Linda T in Atlanta