A seat restraint tensioner for a seat restraint system in a vehicle includes a housing for operative connection to vehicle structure and a movable piston disposed in the housing. The seat restraint tensioner also includes a gas generator operatively connected to the housing for expelling a gas to move the piston. The piston includes a passageway extending therethrough to control a rate of stroke of the piston when the gas is generated to apply a force for pulling-down belt webbing of the seat restraint system.