Only queen is forced to one box. The rest have whole estate 
Operation may have several purposes. For example, swarm prevention and comb building in one package. Another - fast colony growth in preparation to the major flow.
Some swarm prevention methods are based on separation of queen from the brood. An example recently shared by @Doug1 here. General idea - all brood goes up, queen with (or without) one frame of young unsealed brood goes to the first box. Some variants may include filling first box with foundation, putting box with honey frames under brood, etc. The purpose of the exercise is to create conditions that trigger survival instinct and it takes over propagation. That is to say ideas about swarming that colony may have had are being shaken out of it. It works very well. When colony was kicked out of hive and on return sees it completely rearranged they even do orientation flight on some occasions. But more importantly, they start restoration work with enthusiasm of the swarm without swarming.
Fast colony growth.
Having two brood boxes is a good starting point. But when left unmanaged, queen spends too much time looking for empty cells instead of laying eggs. Brood production could be greatly improved if queen has whole box of empty cells available to her. So we give her such box and keep her in it by QX. Then we move box filled with brood up and replace it with another box of frames ready for laying. Then another. When queen fills the box, the first one is already hatched and we swap them. We running this carousel until targeted source of honey begins to flower and after that touching only suppers. The purpose is to have 8-10kg of bees or more ready to work on the major flow. Such colony may bring ~15kg to hive daily while flow lasts.
Then, in preparation for winter we switch back to two boxes without QX again.