The net motion of a swarm is the sum of motion of all members of the swarm.

If the swarm is randomly buzzing, brownian motion, then the sum of vectors nets out to zero.

But if there is some asymmetry in the swarm's vectors, then the overall movement of the collective can become significant.

If there is a small but consistent asymmetry affecting every member of the swarm, the asymmetry might not even be visible at the level of the individual.

Each individual's local context and situation will dominate that small asymmetry.

But nevertheless, at the level of the collective, a massive vector emerges.