Why is "to switch gears" used for "to change topic"?

When you change gears in a manual transmission, you are connecting an entirely different gear to the drive shaft to provide drive.

The idiom refers to this switch to a different, discrete mechanism, not the change of speed that can result. After all, it's quite possible to drive the same speed in different gears.

I couldn't find anything on the etymology of the idiom, but in common parlance let's switch gears or 'change gears' usually means changing the subject. I think this has less to do with the way a transmission works (changing gears changes speed) and more to do with feeling the change of gears. You can really feel gears switch, especially with a bad driver!

When referring to changing the speed of the activity, the gear-idiom I encounter frequently is shift into high gear.