Given the limitations of short-term memory, we externalize our thoughts in order to see them, share them, and scrutinize them. This takes the form of writing (description), drawing (depiction), and gesturing (demonstration).

Through one (or all three modes combined) we communicate or think 'out loud' so to speak. For this reason, it's important to realize the power of writing when writing might be the fastest and easiest way to capture an idea. In another instance, acting out a scenario to 'feel' the experience, makes the best sense. And of course drawing, whether quick basic maps or diagrams, or more elaborate artifacts, helps the designer to 'think' and that's what it's all about.  

Product designers now rely as much on time-based approaches to sketching as they do on perspective or space-based approaches to sketching. Time-based sketches can include storyboards, wireframes, sketchnotes, and paper prototypes, but often begin with non-representational  visuals such as stories, maps, diagrams, and other low-fidelity visualizations.

Even when sketching products, it's good to know the desired outcome in terms of the ideal experience. Processes from Design Thinking like 'jobs-to-be-done and empathy maps work very effectively in framing opportunity. 

Sketching the physical artifact (a smart bike light in the example pictured here) and the app to control it requires a variety of different sketching methods including maps, diagrams, storyboards, wireframes, and perspective sketches.

Often the best way to start such a project, is to map or diagram the user experience. This might involve writing, sketching, storyboarding, wireframing or any number of activities. Like traditional rapid-ideation sketches, sketching for interactions should drive the prototyping process 

Imagining the future is a big part of what designers do today, however, the future changes so quickly and is so complex, that drawings or sketches rarely do it justice- at least initially. Designers were once tasked with "picturing' futuristic cars for example resulting in beautifully drawn and hand-rendered images- usually of a future that never arrived (or was used in Hollywood to make a Sci-fi film like Bladerunner). Today, with driverless cars and AI-equipped robots, it makes better sense to map or diagram the future before sketching it.

Sketching physical artifacts remains central to the profession of product design however, even these sketches must be translated quickly into physical models or prototypes or CAD models. Aligning sketching with these other workflows is critical for the designer.

CAD programs like Solidworks, Fusion 360, and Rhino are common tools for translating sketches into computer models but the sketching process often continues as designers sketch directly on 3D prints or over screenshots of models.