I hope you enjoy this Fly-Catcher device because I’ve never seen anything like it. Watching this device working I understand that it is all about the form of the revolving block with respect the the cylindric cutout that it’s housed in. Essentially, those circled corners of the turning block will be made so that afterward they go by the opening, it forms a cachet with the housing around it. The corners on the bar will then be separated cautiously, so that when the bar keeps rotating to the storage container in the back, it constitutes seals above and below the storage container.
For those living in areas where flies are really annoying this device is the best gadget you could buy. As an alternative, you can find cheaper fly traps on amazon.