As above, there's a whirling vortex of oil mist which is in the crankcase when the engine is running. The "circulation" caused by the spinning crankshaft counterweights within that environment. The windage tray (and related "scrapers") funnel that whirling oil/air mixture back into the oil pan so it's available for the oil pickup to send to the oil pump.
Historically, the B/RB engines were (in their earlier times, from what I heard back then) for "kicking out" the first quart of oil after an oil change. After that, the oil level would remain somewhat constant. In more recent times, I learned that this must have been when the normal B/RB oil change was 6qts w/filter, rather than the later 5qts. Some claim the depth of the oil pans didn't change, just the dipsticks to read correctly for the 5qt oil w/filter change.
I remember that when we got the '66 Newport (when the '67s came out), that we were cautioned to not get excited if the first quart was quickly used, but then the oil level stayed at "Add 1 Quart) from then on out (for the rest of the 4000mile oil change). When the factory windage trays appeared, with the explanation in the magazines, it made more sense then. As it all worked out, our 383 didn't exhibit that "first quart' situation. So all was well . . .
In the later, 1st Gen Chevy LS engines, they ended up adding "windows" at the bottom of the cylinder castings to decrease crankcase windage. Which allowed the engines to rev higher without a power loss. Might not have decreased windage, but decreased the related turbulence by letting the oil/air mixture swirl with less restriction.
Just some thoughts,
CBODY67
