Originally Posted by JAXPax

Seatbelt extenders are NOT allowed on inflatable seatbelts. This is common amongst all airlines... inflatable seatbelts are a relatively new invention having to do with preventing head impact in areas where being flung forward would impact a hard surface or out of control... such as angled seats or in a location where you cannot brace yourself against the seat in front, generally.

A seatbelt extender basically prevents, for lack of a better term, the circuit from being completed between the two ends of the seatbelt that activates it in the event of the appropriate G-force.