Adjustment should not be made during the movement of the car.
To reduce risk of slipping under a zone belt at collision, do not reject a seatback back, 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 rejected a back back, the zone belt can slide off and strongly press at collision 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 completion of adjustment of an inclination of a seatback pressure her a back to be convinced of its reliable fixing.