The B'aht Qul challenge was a Klingon ritual, similar to Terran arm wrestling, practiced to demonstrate physical strength. The challenge involves two opponents facing one another locking their hands together with the backs of their hands touching on a table (or another flat surface). After a count, each competitor tries to push the backs of the opponent's onto the table. He who does so first is declared the winner.
Captain Nu'Daq challenged Lieutenant Commander Data to the B'aht Qul challenge in 2369 after hearing of Data's reputation for great physical strength – Data easily won the challenge. (TNG: "The Chase")