This is a drum key that works like a torque wrench - using the dial on the bottom, you set the torque; when the lug reaches that torque, the top of the key 'snaps' open.
Works perfectly, it's super fast and easy to use. It's important to realise that is sets lugs to the same *tightness*, not pitch - just becasue the lugs are all at the same tension doens't mean they'll all be in tune with each other, especially on a cheaper drum.
With a new skin however, it'll get you in the ball park really quickly. From there, you can use it like a normal drum tune to fine tune things. The socket end is magnetic so it sticks on the lugs, and the grip end is nice and big so even on stubborn lugs it's easy to turn. It might seem a bit expensive, but if you're a drummer or an engineer, you should own one of these.