Diagonal downward cut with the sword, following the line of the kesa (= the front hemline of the jacket).
Unlike with SHOMEN-UCHI or YOKOMEN-UCHI the cut is not stopped, but continues almost down to the ground (YOKOMEN-UCHI is also a diagonal cut, but it is executed at a flat angle sideways to the head/neck).
Search "Basic Forms" page for videos showing KESA-GIRI
