What did it say when you read about this to correct it?
I'm not sure what you are referring to...how to correct a dog not holding until released?
I train mostly by myself, and my method was just to wrap the check cord around my boot, giving a couple feet of slack. This gives the dog just enough slack to start getting a good bit of momentum and get a brick wall at the end of that slack but not enough to hurt it. It's the easiest self correction I could come up with while training with no partner...that way I could work the launcher and the gun during training sessions. If the dog broke at the flush it got a negative association with doing that by hitting the end of that lead sharply, I would then calmly go and pick the dog up, place it firmly but not being mean exactly where it was on point, tell it whoa again, and praise it for remaining on whoa for a while. Then I would release it with my release command and we would go to the next planted bird scenario.
With my whoa training I was releasing it initially by tapping the dog on the head and saying it's name at first...seemed to really make the dog steady until I absolutely wanted it to release, but I've since transitioned away from the head tap because I don't want to have to walk back to the dog all the time to release it and that would prevent me from releasing the dog early, etc.
BTW, I know tons of hunters only want their dogs steady to flush so the dog is hitting that shot bird as quick as possible after the flush. There is nothing wrong with that in my book, I was only answering the question of what we train to. I'm just training to be steady to shot/fall because I thought I might as well and if I'm hunting birds that have a better shot at getting away like pheasants in deep cover, I can always release the dog early by saying it's name at the flush, etc. I figure it's not much energy to say the dogs name and at worst I'm giving the bird an extra second or two head start. I expect the dog to have a good enough nose and marking ability to find that cripple. My pudelpointer will not give up on a dead bird search and has proven to me for the type of hunting I do currently to be extremely good at getting all shot birds. If my hunting preferences or needs change, I'm sure I can easily backtrack and make the dog steady only to flush by releasing it early every time and it would just anticipate the early release after awhile.