Left Feet, Left Pedals
Ever wonder how that soft pedal on the left really works?
On a grand piano, when you depress the una corda pedal (also called the shift pedal), the keyboard moves slightly to the right. This causes the hammers to strike fewer strings on each note. (Most notes have more than one string.) The result is a softer tone, and a different tone color as well.
On vertical pianos, the left pedal doesn't change the number of strings that the hammer strikes. Instead, the pedal pushes all the hammers half way to the strings. Since the hammers have a shorter distance to travel, they hit the strings with less force and therefore less volume.
So on a vertical piano, the left pedal is like an off-and-on switch--press the pedal and the volume drops. But on a well-regulated grand piano, you can use techniques such as half-pedaling to get not only a difference in volume but also subtle variations in tone color.