They're selling a pendulum...... some say they work, I had so-so results with my first try... going to try again with Sexlink eggs next spring so I can know immediately the results...... tie any weighted object on a string and give it a go
You can google how to use them or even search for the threads that have been started here
It requires a belief in the process, but please don't waste your money on that ebay junk
I would be willing to bet that it's the nail on a string trick. I've had people do this to my birds at swaps before. They tie a nail on a string and hang it above the bird's head. If it circles- male, if it sways- female. It's really funny to watch the person's hand when they're holding the string. It's never steady!