Early designs are best considered as sketches. They illustrate the essence of an idea, but have many rough and/or undeveloped aspects to it. When an early design is displayed as a crude sketch, the team recognizes it as something to be worked on and developed further. Early designs are not limited to paper sketches: they can be implemented in a video, an interactive slide show, and as a running system as yet another way to explore the ideas behind a highly interactive system. When systems are created as interactive sketches, they serve as a vehicle that helps a designer make vague ideas concrete, reflect on possible problems and uses, discover alternate new ideas and refine current ones
— Usability Evaluation Considered Harmful (Some of the Time) - Bill Buxton
Sketchiness of tools. Importantly, this is not a matter of leaving hand-drawn marks, though having them helps. This point is more about working with half-formed ideas, and a big part of it is that the Fidelity of The Tool Should be Proportional to The Maturity of The Idea. Basically, computers are great at enforcing rules (Computer Science Positivism), but tools in the earliest stages of thinking, should provide the exact opposite — freedom to poke at the problem from multiple ends, freedom to leave unfinished pieces lying around in your Peripheral Vision, freedom to be messy.