Jack Brownbill it depends which way you are facing, forward or aft.
Forward facing is lap belt and harness fastened, feet flat on floor, legs to 90 degree angle to floor, chin lowered towards chest, hands on top of knees.
Aft facing same as above except head is held up and rested back on headrest.
These are both relaxed positions if it was for an actual brace position then a more pronounced head position is required for both.
Out of interest why do you want to know?