At impact.... impact fix. Once you see the 'lever' of the left arm/shouluder/club, and know that lag pressure, and lag are the goals of the true swinging force (because they give you the most efficient and supported force, with the most margin for error), you can see that to support impact, you must be - behind, under, and inside - the position of the left shoulder at impact, and post impact, the right shoulder replaces the left. Best to stay behind, under and inside the entire swing (which is why Hogan says that you can be under plane and get away with it, but "don't break the glass")
You have to understand the feel of the solid left side, and the right arm extension from the inside.
Split grip drills will really help you get this feel.
Never forget that everything you do in the swing is about impact/separation and supporting on plane force. Set up in the impact position, as routine, or as a drill, your choice - but if you don't fully understand the impact position and lag, you will never hit your best shots.
See the cover of Hogan's five lessons. Allow rotation and a true swinging, balanced, supported, force