You’ll see the pros do this on cobblestone sections in events such as Paris-Roubaix.
Let’s say you’re cruising along in a 53×19-tooth gear. Spinning and grinning. But uh-oh, here comes a gnarly stretch of bumpy, patched pavement.
Just before your wheels start rumbling, shift to the 17 cog. This higher, harder gear increases pedaling resistance and slows your cadence, in effect putting more weight on your feet and less on your seat. You bounce less and your butt doesn’t get as hammered.
The higher gear gives your feet a more stable platform, reducing bike chatter and increasing your control. Pedaling doesn’t feel as choppy. Yet you go just as fast.
Also, you won’t spin out your gear if you need to stand for part of the rough section. When you regain smooth pavement, shift back to your spinning gear and carry on.
This is a good technique, too, when you come upon a loose or slippery surface. A lower cadence in a higher gear gives you more traction and control.