The vertical blind cord is run through pulley opening. You may have to adjust cord length to get tension on the pulley. These are to be used mostly with vertical window blinds. Pulley is spring loaded to keep cord tight.
Spring Loaded Cord Tensioner for Cord Loops – Ivory, Wall or Floor Mount – 1 …
Whether you’re installing draperies, vertical blinds, or continuous corded traverse rods, this tensioner provides peace of mind. ENHANCED SAFETY FEATURE: The spring-loaded design maintains cord tension, minimizing the risk of dangling cord hazards.