The "Speed" node adds a vortex vector.

[tyFlow]

Interesting idea, cgdvs...so basically a force that rotates a particle around the axis of its velocity? The only issue I see with this is that the force itself will change its velocity vector in the process which will change the direction of that per-particle vortex force in the next time step. You would need a way to track the initial velocity, so the resulting spin doesn't affect the future per-particle vortex computation.

You can achieve this with the Script operator already, so you may want to experiment...