Alpha-Nock lighted nock system greatly assists in finding arrows after they're fired. To install these lighting sticks insert the lighted end of LED unit into the open end of the Alpha-Brite nock, then insert the nock and LED unit into the Nock Receiver. After that insert the nock tabs between the receiver’s locking clips until it clicks. When the crossbow is fired, the space between the the light stick and nock receiver is compressed and activates the light. Available in red and green. TenPoint Alpha-Brite Lite Stick Grn HEA-312-3