Probing lenses combine zoomable user interfaces with distortion lenses [Carpendale & Montagnese 2001].
Select the appropriate tool icon from the palette, as demonstrated in the video. A left click activates a zoom-in lens. The lens can then be moved inside the window. The magnified region is called the focus, while the surrounding (non magnified) region is called the context. A second left click will bring the context at the same level of magnification as the focus, thus flattening the lens, whereas a right click will simply deactivate the lens.
Conversely, starting with a right click will activate a zoom-out lens. The focus will remain unchanged, while the context will contain an un-magnified view of the surrounding area. A second right click will flatten the lens in this new context, whereas a left click will bring the context back to its original magnification level (same as the focus).
Whenever a lens is active, the mouse wheel can be used to control the magnification factor of the lens.