Description:High-Visibility LED Bowstring activated on-off switch Breakthrough design. 30 grains total weight half moon shape nocks fits for crossbow bolt size 0.297"-0.302"(7.6mm) ID can be lighted about 48 hours. Method of use: The nock will be lighted by the pressure of tense string. you can use the little tool to push the colorfull part up through the hole when you want to colse the lighted led.very easy and convenient to use.