Forwards is up, up is back, back is down, down is forwards.

It's much easier to reason about when you frame it closer to reality: you're not on a circular path, you're continuously falling, and because you're moving forward, you're continuously missing the earth, with its pull decreasing with distance.

Or in the words of Larry Niven (The integral trees)

East takes you Out

Out takes you West

West takes you In

In takes you East

Down is where the enemy gate is.

So precise, he piss on a plate and never splash.

How related to this is the helicopter 90-degree phase lag thing?