Sit or Whoa to Flush or Flight
I have come to prefer making a game of it that conditions the pup to want to sit-to-flush, rather than sit-to-flush/flight being something denying him what he wants. Which is a practice I came to through the backdoor route of pointing dog training.
My first Chessie's greatest shortcoming was the difficulty I had steadying him and keeping him so, particularly when honoring. So I was receptive to the notion of teaching steadiness from the get-go (rather than again following the traditional US course of building desire to go and then breaking to steady) when I saw it in an article by or about Robert Milner, and that worked out so well with my next Chesapeake that I was, in turn, receptive to Jim Marti's "Burnt Creek Method" of steadying pointing pups before they could develop a chasing habit. Marti steadied his pups with a checkcord, much the same as I've outlined traditional sit-to-flush for retrievers, which made a lot of sense to me, but also still seemed more pressure than I wanted to put on my pups. So when I started the current Brittany, we took a more gradual approach that inadvertently turned stopping at flush/flight into a game for the pup and later proved applicable for reprogramming the Chessie I then had and starting subsequent ones.
The great rub with my route is that it employs as many as a half dozen fly-off birds per session, which pretty much mandates maintaining a loft of homing pigeons or very ready access to wild ones. With the great value of using fly-offs being that they're quickly out of sight and mind, which helps put emphasis on the praise Pup receives for not chasing them, rather than what he's missing out on.
My pups have been taught to heel and sit when we stop, both on and off lead, prior to beginning our stop-to process, so that's old hat, and when I add the toss of a fly-off pigeon to a session of heeling on lead, it's initially just proofing a known concept. During which, I quietly and gently as possible enforce compliance with the lead and then praise, as if Pup had sat of his own accord. All about as positive as can be, short of treating. It's not taken too many such sessions with fly-offs before my pups are not just stopping and sitting on their own but have started anticipating the fly-off and wanting to stop when I reach in the bird bag. At that point, it's become a game between us, with my challenge being to keep them moving and make the eventual toss a surprise and theirs being to show me how fast they can get their butts on the ground when they see it.
Once Pup's hooked on our fly-off game, we advance it to heeling off lead and, when that's down pat, while running free in the yard. All the while keeping it fun by backing up, rather than cranking the pressure up, if Pup slips up. When sit-to-flush/flight gets to the point I can toss birds at the pup and he's happily showing me what an ace he is at stopping (and sitting in the retrievers' case), we've a great foundation that readily transfers not just to sitting in response to self-flushed birds but to steadiness in general. That, and a great tool to return to when steadiness afield falters.