How does a drop down seal work?

A drop-down seal, also known as an automatic drop seal or automatic door bottom seal, activates when the door closes. A mechanism within the seal is triggered as the door reaches the frame, causing a sealing gasket to drop down and create a tight seal against the floor or threshold. This helps to block the passage of air, draughts, sound and smoke through the gap beneath the door. When the door is opened, the seal automatically retracts, allowing the door to operate freely without dragging on the floor.