The thingy they use is a little gadget (it has a name, but I can‘t think of it now) that has a dial on it that indicates the force in Kg that you are sqeezing the handle mechanism. The dial is built so that the highest force is indicated after you let go so that the tester can see your score when you hand it over.
The technique is simple. Stand with your arm straight out from your body so that your hand is level with your shoulder. The palm side of your hand faces forward. Start gripping, and as you do, bring your arm down to your side, while turning your palm in to face your body. Do not touch your hand to your body in any way when you reach the end of the motion.
That simple.