Daffy, where the tether is doubled over and sewn I belive is designed to break, but absorb the initial load.
That is exactly what they are designed to do and the reason I was never tempted to test mine. I want it to stay just like it was designed. BTW, a little traffic safety tip I learned in traffic Homicide Investigation school. Seat belts are designed to have some play in them at high stress from an accident. Once stressed they become weaker so after an accident where they were used, insurance companies are supposed to pay for the cost to replace them.
