When boiling an egg, after the water is boiling, shut off the power (electric or gas). Let the egg sit in the pot of hot water, it will continue to cook.
Those egg gadgets are great for cooking eggs. I like my eggs cooked soft. (not hard boiled)
If you plan to cook the hell out of the egg, you don't need the egg gadget. Don't leave the egg in the hot water for very long, after the water comes to a boil. (probably only 2 or 3 minutes)
If you cook the egg for 5 minutes, it's probably a HARD EGG. The egg gadget is useless, if you want the egg HARD. (cooked solid all the way thru)