I think he means he leaves one side of the box open so he can press it over the cell. You would put the cages over the cells about day 10 (in bees lifecycle) or about day 7 after grafting or making them queenless (after cells are capped). Then, after hatch, you would make up a nuc, leave it queenless overnight and introduce her either in a cage or grandpa Jim says he puts honey on her and introduces her directly.