I understand that, and I have seen this done with just caught swarms before.
They leave the board 4 inches from the hive entrance, in a hive that has built swarm cells, but has not swarmed yet. The bees that were preparing to go with the swarm, will not "jump" the four inches to re-enter the hive. Are these bees that were going to go with the swarm, really young bees that have never flown out of the hive?
What would be the advantage of doing it this way as opposed to a walk away split? Would it be that it mimicks the swarm exactly the way the hive had it planned, and splits the bees accordingly?