I swing past and adjust my speed of swing to the vel. of the target, be it a bird or a clay target.
It just happens naturally, slapping the trigger when the bead touches the bird, carrying on through. The proper lead just happens. If you stop your swing, you miss, just as would happen if you were using the sustained lead. I find that a sustained lead will not work for me any more, especially with the flint lock. With that one, I have to slap the trigger after passing the bird by about a foot. That works out the lead for the slower ignition.
Back when I used a sustained lead, shooting passing birds from above on a hill over the water, we were able to see where the shot was hitting. We were shooting gadwall ducks that were just barely skimming the surface and crossing us at about 60yards out & moving right along, maybe 60kph.
It was easy to figure the lead by how far we were hitting the water behind them. Turned out the proper sustained lead was about 40'. It was a ridiculously long lead, but worked. Incidentally, the gadwal is the best tasting "puddler" duck, better than mallards and teal.
The gadwal is about 99% insectivorous and does not normally eat rotting salmon, like the damn Northern Mallards.