I believe it gets it name as the tail laps over the plate which creates an overhang to shed the rain water away from the side of the building. And that the notch cut into the seat looks like a stair step.
In this drawing the rafter is a 5×5 and the tail laps that over the plate is 1 1/2″ thick and this is the same dimension as the step. Usually they are both the same.
The rafter has been cut back with an arc to help eliminate a “point of fracture” where the roof load could split the timber.
The plate in this drawing is a 6×6.
This shot is of the end of the plate and the gable rafter.