The extender mounts between the compatible lens and body, and multiplies the focal length settings of your lens by 1.4x That’s the good news – the bad news is the 1.4x decreases/narrows your lens aperture setting by 1 stop. The lens aperture still opens to the same width, but the ratio of the aperture opening to the focal length is reduced – by 1 stop.
For example, the Canon 1.4x makes a Canon EF 70-200mm f/2.8 L IS Lens into a 98-280mm f/4 IS lens – and yes, IS still functions normally. A Canon EF 70-200mm f/4.0 L Lens mounted over a 1.4x becomes a 98-280mm f/5.6 lens. A Canon EF 300mm f/2.8 L IS Lens becomes a 420mm f/4 IS lens.