In dynamic light scattering compared to static light scattering, not the angle dependent, but the timely variation of the scattering intensity is utilized for the particle size determination.
On the numerous particles in a suspension, the laser light is scattered in all possible directions. When selecting a direction, the various particles scattered by the light rays interfere with one another resulting in a certain scattering intensity.
Due to the Brownian motion the relative positions of the particles constantly change in regards to one another which alter the inference conditions and therefore the scattering intensity. When the particles move fast (small particles) is the alteration of the scattering intensity subject to fast variations, whereas slow (large) particles lead to slower variations.
With dynamic light scattering the sample suspension is usually at rest. The term dynamic refers therefore not to the sample movement as a whole, but instead to the "trembling motion" of the primary particles.