Lens Shift
Lens shift is generally a feature of high-end projectors or specialist home cinema models. Lens shift has a similar effect to 'Keystone correction' except that the effect is achieved by physically adjusting the angle of the projectors lens to square up the image. This is a better method of producing a square image, rather than using keystone correction as the correction with lens shift is achieved optically (rather than digitally) resulting in no loss of quality
Lens shift can also be useful for fine-tuning the position of the projected image on your screen