Adjustment should not be made during car movement.
To reduce risk проскальзывания under a zone belt at collision, do not reject a seat back back, it is more than it is necessary. Seat belts provide the maximum protection if passengers sit directly and having moved on a seat as it is possible further back. If you have rejected a back back, the zone belt can slide off at collision and is strong press on a stomach. Therefore at collision the risk of a trauma increases together with increase in a deviation of a back from a vertical.
For end of adjustment of an inclination of a back of a seat press on it a back to be convinced of its reliable fixing.