Ozy,try this.
Once you have set your stance,kick your hips back a couple of inches.
This helps align your shoulders and helps prevent the over draw that can result if you just drop your bow arm onto the target from a straight stance while shooting down hill.
The same applys when shooting up hill,after you come to full draw,kick your hips forward a few inches,again it will help align the shoulders and help prevent the under draw that will happen if you just lift your bow arm level with the slope but not your whole body from the hips up.
I personaly belive it's a mistake to say "bend at the waist" because it's not the waist that sets anything,It's really the hips and the shoulders just follow.
Give it a go mate,,the best FITA field archers in the world both refer to it and do it that way,,,,plus I got that tip from the Ozbow web site,,,so it can't be to bad huh.
Go the Blacks,,,LOL.