The author tells us what happens in the middle of P2: “In contrast, the plate collisions in the quiet subduction zones occur between two plates that are moving in the same general direction—the second plate's motion is simply faster than that of the first, and its leading edge therefore becomes subducted. But in this type of subduction, the collision zone moves with a comparatively high velocity relative to the mantle below. Thus, rather like an oar dipped into the water from a moving boat, the overtaking plate encounters great resistance from the mantle and is forced to descend steeply as it is absorbed into the mantle. The steep descent of the overtaking plate in this type of collision reduces the amount of contact between the two plates, and the earthquake-producing friction is thereby reduced as well.”