You lean forward... then lean the top of your body back again.
This is Joe Gibbs - easily the worlds best heavy bow archer, capable of shooting over 200lb at the moment showing the perfect form for really heavy bows. Once the back muscles have got used to the weight this exaggerated form can be reduced quite a lot, but if you're struggling to get fully inside a heavy bow, it's really the best way to do it.
You can see in loefflerchuck's photo of him drawing the bow that his body is very upright and the elbow is really high which means all the work is being done by that arm instead of the upper back. Leaning into the bow isolates those upper back muscles, and they're massively stronger than the arms.