You rush your mom or whomever off the phone in some less formal syntax (“Yo, I’mma holler at you later,”), hang up and get back to work. Then you look up and you see your co-workers looking at you and wondering who the hell you’d morphed into for the last few minutes. That right there? That’s what it means to code-switch.
My dad and my sister are experts at (subconsciously) stepping up the Southern accent ever so slightly when the situation calls for it. I’m pretty sure I do it, too, but it’s quite possible everyone sees right through me.