Less intense than the experience of "blacking out" when drunk and not remembering portions (or all) of your night, "browning out" occurs when you don't remember something until someone brings it up. It's not a complete blackout, but partial, because you remember once someone refreshes you.
I didn't even remembermaking out with Bryanne until J-Lo told me (it must not have been that good)! I definitely had a brown out last night.
Those four words roll off the tongue as a lighthearted way of asking "What's up? What's next?" And that tripping lightly stuff makes sense, since "How now, brown cow" has its origin in elocution, where the phrase was used to demonstrate properly rounded vowels.
Ron: How now, brown cow?
Paul: Not much. Just on my way to catch a movie with my girl.