I think I'm better understanding the weak/strong coupling idea.
His lumbosacral junction seems fairly close to the points of the hip, as in a shallow triangle. His back is roached, though not alot, I think.
He is 17 years old, TB, and fairly new to me (just 2 months). He had evented previously, and I am into riding for pleasure/handy horse/flat/trail/some jumping.