Crew Hauling a Line
To be able to handle heavy weights, or tighten lines without winches, crew members had to pull together. Raising and reefing or shortening sails, trimming or hauling in control lines (sheets) to reef a sail, or hauling in some other of the hundreds of lines aboard a large sailing ship were all done by hand. A shanty sometimes helped coordinate the pulling.
Here in a photograph that may have been posed, one sailor looks at the camera rather than at his work. He has a sheath knife and as does the sailor whose back is to the camera. A knife and other tools worn at the small of the back were not likely to foul in the rigging when climbing.