What Karnak said.
It's the lap-strap that holds you in the seat while you're maneuvering..even upside down you feel the pressure far more on the lap than shoulders.
So your butt is in the seat, on hand on the stick, other on the throttle...and those won't pull out of the airplane... so you're in a "three-point" configuration.
The shoulder strap doesn't have to be locked down tight.
In fact, I think you'll find in MOST aircraft, particularly the US anyway, that the shoulder straps have a two postion lock switch. I'd have to check to be 100% positive.
One is an inertia reel that allows movement unless under sudden g... like your car works... and the other position locks it tight where it is.<EDIT> Well, it locks the inertia reel, allowing the shoulder belts to retract but not extend any further.
[ 06-20-2001: Message edited by: Toad ]