For practical purposes, the stretch is 'extending your left arm with your right' directly toward the ground in the direction it is pointing when viewed from down the target line at impact fix.
Put another way, if you draw a line from the butt of the club up the underside of your left arm, you want to stretch your left arm down that plane. It can be very useful to think in terms of this "under the arms at address" line/plane.
See the drills section, the 'variation on table top drill' at the bottom to give you a clearer picture of what I mean:
Hybrid Mode
