One correct feeling; Back exercises

After a couple weeks off, we had a lesson with John today.  After some warm up, John's first instruction to me was not to let Charlie just puke over the warm up fences.  Our second task was to switch right to a hefty* line and oxer.  After a few rounds of that, Shannon rode some other lines, and then we did a new set of lines that were even heftier.*  Those four fences were amazing.   Afterwards, John said that's how to ride every fence, no matter how big it is.  He said when they're bigger and I'm a little nervous, I naturally sit up and collect Charlie, like I should for every fence.  Instead, when they're small, I let Charlie take the lead, and then he puts forth minimal effort and they're uncomfortable pukers.  I agree that Charlie is easier to ride when he's bigger and the fences are bigger, but I'm still really wrestling with how to get him big without letting him get flat. Shannon said she noticed that I ride with my heels just dug into his side, so she thinks that I can't use my hips and upper legs very well and that I need to get him in front of my leg and responsive first.  So we'll see - it's a lot of interlocking pieces that I feel like I am close, after a lot of years of sweat, to figuring out.
The other thing I asked John about was exercises for my lower back.  He recommended laying on a pillow, just to stretch it out, and then to use my pecs to pull my shoulders back (not lean backwards and roll my shoulders back), and then third to think about pushing OUT right under my rib cage, which also pushes my back out.  He said it's actually the back under the shoulder blades that needs to be improved, not the low back where it looks the worst.
So I've got the good feeling imprinted in my head now, the next task is just cementing how to get it and then consistently get it.  Baby steps.  But man, Charlie feels like a dreamboat when I get it together.