I'm not sure if I am recalling the correct device, but --
 
The board is a piece of wood sloping to the ground, which is set up in
front of the hive.  It is positioned to leave a few inches-gap between it
and the hive entrance.  The bees are then shaken off the combs at the base
of the board.  When they run up its surface, the older bees go straight
across the gap into the hive, but the younger bees (and queen?) end up
clustering at the edge of the board, like a swarm.  Then they can be taken
away and hived elsewhere.
Or that is the jist of it, I think.  Can anyone elaborate?
 
Thanks muchly,
 
Joel Govostes
Freeville, NY