On a long climb, you're better off staying in the saddle to conserve energy and keep a steady pace. When you stand out of the saddle, it's more difficult aerobically and your muscles will fatigue quicker. While it can increase your power and speed for a short period of time, it's not sustainable for really long efforts.
However, there are times when you standing out of the saddle will give you some benefit. If there are sections of steep road or the competition in a race is close near the finish, a surge of power and speed might be necessary. The climb might also be really long, and standing on the pedals every once in a while can provide mental and physical relief from the change in position.
Regardless of why you do it, you should take advantage of your time out of the saddle. If done correctly and for short amounts of time, you can make the added work on your legs less difficult and climb more efficiently when you do decide to stand.
Vertical Motion of the Pedal
To understand how, we need to first think about the vertical motion of your pedal through the crank cycle. Let's assume we have a 170 mm (or 17 cm) crank arm. Between the time when your crank arm is pointing straight up in the air to the time that it's pointing straight down (a.k.a., top dead center and bottom dead center, respectively), there is a 34 cm vertical displacement.
When you're sitting on the saddle, your leg does all the work to make your foot push the pedal through those 17 centimeters. But wouldn't it be great if the crank would do you the favor of moving up a little?
That's exactly what you can make it do when you stand on the pedals. Just be sure to emphasize what already comes naturally to you—and rock the bike side to side.
How Rocking the Bike Helps You
The diagram below exhibits a rudimentary head-on view of our bike.
On the left, we see a steadily vertical bike. On the right, we have a bike that starts 15-degrees to the left of vertical, then goes 15-degrees to the right (the angle has been exaggerated here for visual purposes).