The audience can neither look forward nor back, and it cannot compare two aspects of the talk except mentally. A sequence of slides is also a linear presentation. The audience sees only one slide at a time and, assuming that the slide contains text, they read it from top to bottom. Comparisons are possible only within one slide or, to a limited extent, between two successive slides. There are several ways that you as a speaker and that your audience can interpret these two visual streams.
Examples are two images, two equations, or two algorithms.
To use this power effectively, make sure you are comparing apples with apples. You may also use the optical pointer, but first read the advice in item 4 below. An image and an elaboration. Examples of elaborations are detail images, a cartoon drawing, a performance table or graph, or a text explanation. This scheme works especially well if you have several pairs of images and accompanying elaborations.