This was invented by G.F. Taranov, a Russian Beekeeper, and consists of 2 sheets of plywood (or similar) set with open end propped by struts. A rough length of wood (25mm. square approx.) is fixed under the top board 100mm from the open end - this forms a focus for the clustering bees.
The board is placed with the upper edge 100mm. from the alighting board/hive entrance and set at the same height. A cloth is placed to cover the lower portion of the board. Then frames are shaken onto the lower end of the board. The bees will walk up to the top edge of the board where "swarm" bees will go into the dark region under the board and cluster, along with the queen. The non-swarming bees (mainly older) will fly across the gap back into the hive, which should be left with a frame containing 1 or 2 queen cells to produce the new queen (this frame should have been brushed clear of bees, NOT shaken).
See Dave Cushman's website for pictures for construction details.