1. The chain can bend unidirectionally. It stretches out while open window and retract to opener inside after closing windo. Don't support close/open by pushing/puling directly. 2. Double layer chain, more and not easy to break off. 3. Equipped with overload protector and can open and close for 30000times. 4. Smart design with strong funciton. 5. Locking force reach 2000N. Push/Pull force 400/400N.