Both the seat and seatback pull air through the perforations, but the cooling effect is minimal to nothing. I don't feel a thing on mine, but Tesla assures me the ventilated seats are "working as designed." Too bad it wasn't a better design.
Really frustrating that Tesla feels the need to reinvent things that already work just fine in other cars, only to arrive at a less satisfying solution many times. (Just look at Tesla's crappy phone/music interface and functionality, or its disastrous 2nd row Model X seats which don't fold and can't be controlled reliably.)