Achieving the negative down swing plane

I'm not sure what steep is But for me, +ve is where the down swing essentially comes over the top of the backswing plane, causing the out-in/slice that 90% of us have. The goal is to be 0% on plane, but apparently most pros have a -ve downswing, so a -ve downswing is where you come under the backswing plane, to achieve in-out/draw. Bizarrely I can do it on a practice swing easily, but put a ball there and I suddenly go slightly positive again.

I have noticed that momentary pause at the top really helps (by luck really), but that diagram really helps knowing what the position goal is...will try at the range tonight.