This term has always given me grief in that I struggled to understand what it was and how to achieve it. Tonight I may have stumbled on how it's achieved.
I can usually get my back tension set when I'm shooting and if I keep it I will hit what I'm shooting at or at the very least I'll be vertically in line with it. But I'll describe back tension as being on a knife edge. Relax one iota and I lose it. I'll come close to my intended target but I'll be off to the right or left. I usually shoot 50 lb so tonight I picked up a lighter bow (42 lb) and shot for a while. While I was at full draw, back engaged, I tried applying just a smidge more rotation to my draw elbow or stated another way just a smidge more back tension. Every time I did this the arrow flew true. There was little if any additional movement, just a small increase in tension. Does this sound like expansion????