These japa counting beads are small beads on a string that one attaches to the outside of the bead bag
Initiated devotees in the Hare Krishna movement make a vow to chant at least 16 rounds of the Hare Krishna mantra daily on japa beads. Each "round" of the Hare Krishna mantra is chanting: Hare Krishna Hare Krishna, Krishna Krishna Hare Hare/ Hare Rama Hare Rama, Rama Rama Hare Hare, 108 times. So to keep track of how many rounds of japa we have chanted we need some counting system.
The traditional way of keeping track of the number of rounds of the Hare Krishna mantra we have chanted is to use "counting beads." These japa counting beads are small beads on a string that one attaches to the outside of the bead bag and each time he chants a round he moves one bead down. This way there is always an accurate count of how many rounds of the Hare Krishna mantra have been chanted.
The system is there are 20 beads. So one separates it into two sections. One section has 16 beads and the other section has 4 beads. This enables one to use the 20 counting beads to count up to 64 rounds of the Hare Krishna mantra.
Although Hare Krishna devotees only vow to chant 16 rounds of the Hare Krishna mantra daily, that is just a minimum. Many great devotees in the past have regularly chanted 64 rounds of the Hare Krishna mantra daily, or even more. So if you want to chant more than 16 rounds when you come to the end of the first 16 rounds you move one of the four beads down. Then when you finish the next 16 rounds you move another one of the four beads down. So when all four beads are down you have chanted 64 rounds. If you want to chant 128 rounds you can start moving the four beads up again.