Nockturnal Shift FIT Lighted Nocks use a string-activated linear switch with a piston-driven assembly to trigger the LED on release. Super-bright LEDs and lithium batteries provide long runtime, with waterproof and shockproof construction. The Shift Fit Universal model uses FIT technology and is compatible with X, H, S, and GT arrow sizes. Each nock weighs about 25 grains and is offered in a 3-pack with over 20 hours of battery life.