Two-Thirds of aerodynamic resistance is created by the rider and only 1/3 from the bike. So even small gains here can make a big difference. In fact, testing from the San Diego windtunnel shows that the Cervélo Aero Race Team Jersey saves 10 watts when riding with your hands on the hoods at 40 km/h. That translates into about 36 seconds saved every hour. But since drag is exponential, that means you get double the savings at 57 km/h, and 4 times at maximum sprint speed of 80 km/h.