Nose down or inline relative to forearm is a matter of how the disc is seated in your grip/hand/palm/fingers and how much ulnar deviation/bending the wrist down you use. This doesn't guarantee the nose is down to the trajectory though or nose down through the apex of the flight which are ultimately more important than the setup.
If you stand sideways to your target with closed shoulder and aim your arm/disc straight out to the target and have the disc level... now if you supinate your forearm palm up(wrist rolls over) it tilts the nose up - opposite side of disc from your palm. If you pronate your forearm palm down(wrist rolls under) it tilts the nose down to the trajectory. The nose is basically on the opposite side of the disc from your hand as the disc pivots out and releases from your hand down the trajectory.
Pronating palm down also allows your arm to fully extend more safely underneath your shoulder and you can push the disc targetward with your thumb with ulnar deviation(sideways karate chop or hammering motion) rather than wrist extension(backhand pimp slap). I don't really flex or extend the wrist much during the throw, and there's really not much power in your wrist extension and lots of top throwers will say their wrist is locked and some will even say it's straight. If you resist/load against the disc flexing your wrist you can get a tendon bounce or plyometric bounce/spring releasing.
https://www.youtube.com/watch?v=qGGYsSUGKXk#t=4m
I also get some kind of nerve pain when trying to extend the wrist with it already deviated a lot, so I actually keep the wrist fairly neutral/slight deviation and flex, but arm supinated a good amount to keep the disc level in the backswing and then pronate and deviate through the hit/release with the tendon bounce and followthrough and the disc should maintain level to swing plane throughout all this. It's all vey much like using a hammer sideways and keeping the head of the hammer on plane to the head of the nail and pounding it through flush at the very end by releasing the head of the hammer.