Yes dropping your hips too. I prefer a straight Bar myself lying on a flat bench not across it. really hits my lats.. i think he was doing it for chest. it does not matter chest and back are hit. An exercise which really gives not only volume for some ( i do not believe it Stretches the rib Cage ) but also a great tricep workout. Include a press ( Pullover and press) and itīs one of the best movements around.
Is it a chest or back dominant exercise? I think that depends on which phase during the ROM; the deepest part pec dominant and the upper part more lats involvement, while the serratus is activated through the whole ROM (at least, that's how I feel it).
Pullover/press.. I've heard about it, but never tried it; I'm more a fan of single movements.