This is a picture showing some slate work with a wall cricket detail done in copper. This job was done on a historic house in West Point, Va.
Notice that the solder tracks are on top of the new seams. The iron is held on top of the seam to draw the solder up the roof, and into the joint. More soldering details are here: Soldering Details
Here's another cricket, with a chimney thrown in for good measure, on the other side of the house.
The pieces of copper visible in the slate area are bibs, used to hide nails when doing repairs, or just inserted when fixing an undetermined leak, not visible as a broken or missing slate.
Albert's Roofing Repairs | Slate and Window Flashing Co., Rich., Va.