No idea and never heard of this in the past. I suspect that if this was replicated over many different homes, we'd find that in fact it doesn't work. I have no idea how the bag is hung on in front of the door, but I could see how this may influence your observations. For example, if it is placed in front of a door in such a way that use of the door disturbs or dislodges the bag then the door/entryway is used less; thus fewer opportunities for the flies to enter. This method seems to be a very unreliable method to keeping flies out of a home. I spoke with several other entomologists and they felt the same as I.