What it is
This is also a very broad strategy. On the one hand, it can mean that while studying the textbook or article, you create an image in your head of what you are learning. On the other hand, you could also actually draw what you are learning, or create schematics or overviews. Again, the effectiveness of the strategy depends on the manner in which it is applied.
How to do it
One important distinction in this category is whether you are visualizing something in their head for the sole purpose of remembering exactly what you are picturing, or whether you are using any kind of visualization to structure your knowledge and actively process it further. The first approach is not very useful, and therefore we will not focus on how to do that here. The second approach can, in fact, be useful, where you structure your knowledge by drawing a process, combining or contrasting ideas in a mindmap, for example, or creating graphs or diagrams. Try to come up with different ways to represent the information visually, for example, an infographic, a timeline, a cartoon strip, or a diagram of parts that work together.
Does it work, and why?
Its usefulness depends completely on the manner in which it is done.
Again, as stated before, the first approach is not useful, especially regarding understanding. The second approach may be useful if combined with explaining (elaboration) and/or retrieval. This is called the dual-coding technique: when reading a text, make a visual out of it (graph/diagram/mindmap). When studying the graph or visual, explain it in your words. Combine it with retrieval practice and try to write and draw from your memory. That way, you make visualization more effective.
One very common learning myth states that some students are visual learners and learn better with visualization. However, there is no scientific evidence that there is such a thing as a visual learner! Some students might prefer doing it, but the essential ingredient of visualization to work is that it is done in an active way (by elaboration or retrieval).