Can we just keep on going for a moment with this sitting on the whistle thing?
IMO: Timing with the collar correction is pretty important. We want to convey to the dog that the whistle is not a correction, it's a command, the running after the whistle is what is getting corrected. So to put in useable terms, send the dog to the back pile - WHISTLE - dog continues on towards pile - COLLAR PRESSURE - dog stops, sits - PRESSURE OFF - Whistle again - "verbal praise" and then a cast. *The pause between the whistle and the correction should be quick but a fair enough time for the dog to react to the whistle. You must read the body language of the dog to see what his/her intent is upon hearing the whistle.
Loopy or slow sit dogs get the same treatment above. Dogs making an effort to stop momentum and sit do not get any pressure.
I think doing it this way conveys the right message and results in a dog with a much better attitude on being stopped. I want a dog hearing the whistle, spinning to stop and looking at me with the expression, "Where to now, boss?"