Complex sentence: They have pleated mesh (polyester, polypropylene or fiberglass) that folds back into a small cassette; it does not have a return spring and when closing or opening the net it can be stopped in any position.
Split into simpler sentences: They have pleated mesh (polyester, polypropylene or fiberglass) that folds back into the small cassette. It does not have a return spring; when closing or opening the net it can be stopped in any position.