These have been around for a while now and if you hunt around you will find them in other place but usually cost more.
I was thinking the same as you do these work or is this a con. YES they really do work.
Works on hatching eggs, day old chicks and the older chickens aswell.
Swing one way for male and they other for female and same for hatching eggs but if nothing then not fertile.
All explained in the instructions that you will be sent.
This has been 100% for me but im sure nothing can be 100% all of the time.