A knurling tool is used to press a pattern onto a round section. The pattern is normally used as a grip for a handle. Apprentice engineers often manufacture screwdrivers. These have patterned handles, to provide a grip and this achieved through the technique called knurling. The pattern produced is called a 'knurled pattern'.
This diagram shows the knurling tool pressed against a piece of round section steel. The lathe is set so that the chuck revolves at a low speed. The knurling tool is then pressed against the rotating steel and pressure is slowly increased until the tool produces a pattern on the steel.
The automatic control lever is engaged which starts the automatic traverse of the saddle. As the saddle moves along the bed of the lathe the knurled pattern is pressed into the steel along its length.
If the traverse of the lathe is stopped and then reversed a diamond pattern is produced.
Sorry to nitpick, but the diamond pattern is created on the first pass based on the the grooves on the two knurl wheels having different orientations.
Even if you had single knurl wheel tool (which I've never seen), reversing the carriage would not result in a diamond pattern. The grooves on the wheel present in the same orientation regardless of bed travel.