The closest I've ever come to a ground loop was landing on tarmac with a skid. Even though it's free-castering, the tailwheel I have now on the same aircraft is clearly doing a lot to improve directional control. Don't ask me too closely about the dynamics of it!
On most tailsprings, putting weight onto the tail will change the angle of the caster pivot which does improve directional control, though it may not be good for the airframe/tailwheel assembly.