Just to be sure I understand:
There is a "push", with the offhand, on the forestock/rail but the "pull" essentially comes from the "pulling inward" of the elbow and shoulder into the body and spine/back?
With the offhand over the rail, it's still the same "push-Pull" as modified by your technique, the offhand is just in a different position?
Also, for clarity, is the advantage of the offhand over the rail simply that it takes less room in cramped quarters (the elbow is made to come inward a lot more, allowing better maneuverability in tight spaces) compared to the usual method?
Finally, as @SouthNarc asks, what's the pressure, if any, on the cheek?