These were advertised alot during early 70's. They are plastic or wooden "screens" (grates) in the shape of a regular frame, say 1/8 - 1/4" thick. The mesh is such that the bees can pass through them. What you do is reduce the number of frames to 9 (or 8?) and hang one of these dummy grate-frames in between each pair of combs. The idea was to increase the space between comb surfaces without letting the bees construct anything in that space. There were big claims that this was the answer to all your swarming worries, by giving the bees more room to cluster & work between combs. In practice they didn't work consistently and the idea was abandoned from what I remember. Gleanings ran an article or two several years ago that basically said "they don't work - don't bother." BTW they were marketed as "No-Swarm Cluster Frames," and they were pretty expensive for what they were.