SEATBELT DIDN'T TIGHTEN UP INITIALLY DURING A WRECK CAUSING MORE EXTENSIVE INJURY. THEN TIGHTENED UP SNATCHING PASSENGER INTO SEAT FROM MIDAIR CAUSING ADDITIONAL DAMAGE. VEHICLE WAS GOING STRAIGHT AND WAS HIT BY A CAR WHO RAN A LEFT TURN ARROW ON A CITY STREET. WRECK WAS UNDER 50MPH BUT PASSENGER HAD EXTENSIVE DAMAGE DUE TO SEATBELT MALFUNCTION. .