This extender mounts between the compatible lens and body, and multiplies the focal length settings of your lens by 2. That’s the good news – the bad news is the 2x decreases/narrows your lens’ aperture setting by 2 stops. The lens aperture still opens to the same width, but the ratio of the aperture opening to the focal length is reduced – by 2 stops.
For example, the Canon 2x makes a Canon EF 70-200mm f/2.8 L IS Lens into a 140-400mm f/5.6 IS lens – and yes, IS still functions normally. A Canon EF 70-200mm f/4.0 L Lens mounted over a 2x becomes a 140-400mm f/8 lens. A Canon EF 300mm f/2.8 L IS Lens becomes a 600mm f/5.6 IS lens.