When we introduce a queen into an established colony, workers might ball and kill the new queen. Push-in cages help protect a queen. When we introduce a queen in a push-in cage, workers will feed her through the cage screen, and she is free to lay eggs in the available cells, but is safe from balling inside the cage. The other bees can smell her QMP (Queen Mandibular Pheromone), which is strongest when she is laying eggs, and are more likely to accept her. This method can be useful when introducing a queen to a colony with laying workers.
Purchase our premium online course How Is My Queen? to learn more about using push in cages, introducing queens into established colonies, and fixing laying workers.
This video from our YouTube channel is another good resource.
You can make your own push-in cage using the method described in these videos.