They had a total of 6 twisting entrances in three pairs with one leading into upper box and one into lower. Close the top one off and open the bottom one to bleed the foragers who were using this entrance into the lower box to keep a decent sized workforce. Open another top one on a different side for new foragers to re-orientate to.
Something you can't do as easily with a single entrance.
But it's an unnecessary faff. One door, one principal intervention to confuse/redirect to the new entrance, job done. No fiddling, no trying to remember whether it's door 3 on day 6 or door 6 on day 3, etc.
Snelgrove was reputedly a believer of the Gerstung theory which (wrongly) proposed that the trigger for swarming was that house bees automatically produced brood food; an excess of house bees meant that there was an excess of brood food... in order to use up that brood food, first the workers raised drones (because they ate more than workers), then if there was still excess they raised queens (because they ate even more than drones).
Appealing idea, but wrong, but this is what is behind Snelgrove's repeated and unnecessary bleeding off of workers from one side of the split to another. One major depopulation was all that was generally required - and that is achieved by a simple (and single) re-direction of the flying bees in most artificial swarm methods.
This all assumes that the swarming triggers are congestion and/or queen substance production/distribution. If they are inherently swarmy bees they may still try to go anyway...
Snelgrove's systems do work, generally, but they are the equivalent of turning right three times in order to turn left. Snelgrove still has traction amongst beekeepers because he produced a detailed step-by-step system and a gadget. To the beekeeper repeatedly bemused or caught out by swarming, it looks like salvation because it's clearly more complicated than all the other methods which don't seem to be working...
