Counterflashing is a piece of flashing that covers another piece, allowing the lower to be replaced as needed
Counterflashing is common on chimneys to cover the upturned leg of the step flashing woven into shingles. It is also a good idea on roof/wall intersections to allow step flashing replacement during a reroof.
In this video, Kirk Giordano adds a z-style counterflashing over a roof/wall flashing to allow the homeowner/GC more flexibility in the unique situation this particular house offers.
When replacing the roof, you shouldn't have to break into the wall stucco, so creating a flashing in two parts adds options for future home improvements.
Kirk also talks briefly about nail types. He is using a ring-shank nail with a rubber washer under the extended head to hold the roof wall flashing to the roof and to hold the nail to the substrate.
