Basically, the FumeFX Follow Operator forces particles to move in the same way as fluid does throughout the simulation. This will makes it seem as though the particles are part of the fluid, and can be used to add detail to the simulation. This operator only acts on particles that are inside the fluid - all other particles are ignored.
This option is on by default. It will make particles move in the same way that fluid did during the FumeFX simulation.
Velocity multiplier - If you change multiplier from the default value of 1.0, particles will move respectively faster or slower than the fluid, but still in the same direction. Negative values are supported, so particles can move in opposition to the fluid.
Influence % - refers to the percentage of FumeFX's velocity (multiplied with the above multiplier) blended over the existing particle velocity in one frame. Thus 100% means that the particle follows FumeFX velocity completely; anything less gives wind-like effects.
This option is on by default and it will orient particles along the fluid flow direction.
Max Ang. Vel - The Maximum Angular Velocity value can limit the turning velocity for the particles. This value is set in radians per second.
Out Of Grid - If a particle leaves the FumeFX grid - whether permanently or temporarily - you can choose to delete it. You can also delete particles that enter areas in simulation that are occupied by an object. If the particle is not deleted it will continue its movement uninterrupted by FumeFX. Refer to the Particle Flow Follow operator for more information.