1. In an transmission (car or otherwise), the activity of changing a gear to run the system optimally.
2. Used to describe the activity of having vaginal sex and then casually moving to anal sex. This activity generally surprises girls!
Girl1 Hola chica!
Girl2 My BF swiched gears on me last night.
Girl1 How did that go???
Girl2 I was mad at first, but I got used to it. The best part is when he nuts in my ass, I can't get pregnant! My butt is a bit sore though.