The Ja'chuq is a Klingon ceremony that is sometimes part of the Rite of Succession. The Ja'chuq was used in the ancient rite, and rarely used in modern times. However the Ja'chuq can still be used as part of the Rite if desired by the Arbiter of Succession.
In the Ja'chuq the two strongest challengers would each have to, among other things list the battles they had won and the honor they had gained in the process - proving to the Klingon High Council why they were worthy to lead the Klingon people.
The Ja'chuq eventually became obsolete and was rarely used in the modern rite. In the modern rite, all that was required was a declaration that two challengers had been chosen. However when K'mpec died, the chosen Arbiter of Succession, the human Captain Jean-Luc Picard, decided to require Gowron and Duras to undergo the Ja'chuq in order to delay the process so that the crew of the USS Enterprise (NCC-1701-D) could investigate possible Romulan interference in the Rite of Succession. This marked the first time that the ancient ways were used in quite some time. (TNG episode: "Reunion")