What's happening here?

This example demonstrates the particle system with UIEmitter. Particles rise from the bottom of the screen with physics-based behaviors including gravity, velocity damping, and rotation.

const emitter = new UIEmitter(
  layer,
  [
    new UISpawnRectangle([-960, 0], [960, 0]),
    new UISpawnRandomRotation([-Math.PI, Math.PI]),
    new UISpawnRandomTorque([-Math.PI, Math.PI]),
    new UISpawnRandomScale([1, 10]),
    new UISpawnRandomLifetime([10, 20]),
  ],
  [
    new UIBehaviorDirectionalGravity({ x: 0, y: 50 }),
    new UIBehaviorVelocityNoise(1, 200),
    new UIBehaviorVelocityDamping({ min: 0.25, max: 0.75 }),
    new UIBehaviorTorqueNoise(1, 100),
    new UIBehaviorTorqueDamping(0.25),
  ],
  [
    new UITextureRenderingModule(dustTexture),
    new UIRenderingColorOverLife([
      new UIColor(0xfc9337, 0.25),
      new UIColor(0xb77a59, 0.25),
      new UIColor(0xb77a59, 0),
    ]),
  ],
  { expectedCapacity: 1024 }
);

emitter.play(16);