What is the Zoom Torque switch?

The Zoom Torque switch changes the resistance of the zoom ring. T (Tight) sets a greater zoom ring torque and helps prevent the lens from extending under its own weight when it is pointed upwards or downwards. S (Smooth) sets a lower zoom ring torque, allowing the lens to zoom in and out smoothly.
For added convenience when carrying the lens, the zoom ring can be locked at wide end.