Before the Dark System or Light System, we only had moderate control of the molt of flight feathers during racing. One option that I have used for the last 50 years continues to be used today by many. You can delay the molting of a flight by removing 1/16 of an inch off the tip of the flight that you do not want to be molted. This must be done as soon as the previous flight is molted or better yet before the previous flight is molted.
Here is how it works. Say for example your pigeon just dropped the 7th flight and the bird has big race coming up in two weeks. You want as full of a wing as possible for the race. Worst case is the 7th flight is half grown in and the 8th flight drops leading to a big hole in the wing. If you cut 1/16 of an inch off the tip of the 8th flight the pigeon will grow in most of the 7th flight in before dropping the 8th.
If you are really paying attention and managing, you would have taken the tip of the 8th flight before the 7th flight even dropped. In some cases, the pigeon may grow in the entire 7th flight before dropping the 8th flight. Gives you a good chance to have a nearly full wing for the race. This can be used for any primary flight in the wing and will also work with the tail feathers.
In the photo of this young bird National winner you can see the fancier clipped the tip of the second flight. The plan was for the pigeon to hold the second flight as long as possible and have the bird fly with at least nine full flights for a few races.
Why this works? Mother Nature gives our pigeons every advantage possible. If the pigeon is living in the wild and has a broken 8th flight it would be at a huge disadvantage to drop the 8th flight if the seventh flight was not completely grown in.
By removing the very tip of the flight it triggers the pigeons system to think that the flight is broken. This broken flight now delays the start of the new feather that will push the old feather out from the quill. Just another built in system to give the bird the best chance of survival.