The standard airline brace position is designed around a lapstrap only. Virtually all light aeroplanes have a 3-point harness similar to a car seatbelt. The upper torso will be restrained however - and indeed can't reasonably be asked to move forwards, that would go against the basic geometry of the harness.
Therefore the important things are limbs. Most teaching would be that the feet should be firmly flat on the floor, the hands on the knees, and the chin on the chest. That way there's as little potential for limbs to flail as possible.
G