Slow shutter - 1/15 or slower. Static camera (or near-static). Moving subject. They blur. The world around them stays sharp. A person walking becomes a smear of motion through a still environment.

Works well with crowds - one person moving differently from those around them, or everyone moving at once.

Watch for: complete subject abstraction. You want enough of the subject visible that they read as human. A full blur is often less interesting than a partial one.

The point: blur communicates time. A sharp photograph freezes a moment. A blurred one shows duration.