Watch a pro race video and you'll see the guys riding no-hands while they peel an energy bar or stuff a rain cape into a jersey pocket.
Most of the time they sit tall during this sleight-of-hand. But sometimes in certain conditions, you'll see them riding no-hands with their backs nearly horizontal. They're as low as if they were still holding the handlebar.
They stay low to remain aerodynamic and save energy compared to sitting up, catching the wind with their chest and pedaling harder to keep their speed. In the tight quarters of a racing pack when the crunch is on, disrupted rhythm is a problem.
Also, staying low keeps their hands closer to the bar. They can grab it quickly to brake or steer around a problem.
If you're comfortable riding no-hands in the conventional posture -- sitting straight up --it's easy to do it in a lower position.
Learn this technique on a low-traffic road with no cross streets. Ride at a moderate pace and shift to a rear cog about two teeth smaller than normal for the conditions. (Example: If you're on the 19-tooth cog, shift to the 17.)
Slide back in the saddle. Take your hands off the bar but don't sit up. Keep your back at the same angle as during normal riding. Keep your hands near the bar so you can correct any wobbles.
Ride that way for 20 or 30 feet. Then grip the bar, ride normally for a few more seconds, and repeat. Work on keeping your hands off longer each time. Soon you'll get the hang of the balance and coordination it takes.
Three tips will make this no-hands technique easier:
Keep weight rearward. Many pro riders have their saddles slammed all the way back. This puts weight to the rear and makes it easier to support the upper body while pedaling. Make sure your saddle position is correct. If you find it can go back farther, it'll help you ride no-hands this way. Of course, don't compromise proper bike fit just for this technique.
Use a bigger gear. Your legs need something to push against so they can share the load with the lower back. A larger gear and slower cadence make things easier. If using the next smaller cog doesn't help enough, drop onto the one after that.
Develop a strong low back. When you try this no-hands posture, you'll quickly find out if your core muscles are weak. If they are, a program of back extensions and crunchers will help.
Source: RBR