Modern DSLR lenses don’t usually have aperture rings, and opening and closing the aperture is the camera’s responsibility. If for some reason you need to keep the aperture blades locked in a certain position, the “lens twist trick” can help you do so. Simply untwist the lens from the camera while holding the depth of field preview button.
One practical use for this trick is time-lapse photography. Cameras don’t always close the aperture to exactly the same size every shot, and the slight variation can cause a flicker in the resulting time-lapse video (a problem called “aperture flicker“).