This doesn't always get a lot of traction, but I have medically diagnosed ADHD. The meds cause terrible and dangerous side effects for me, so I cannot take them.
I decided to actively train my focus in my early twenties. Over time I got better and better at it. I require a few prerequisites: well rested, recently exercised, not too hungry, have no meetings coming up, and keep my phone away. I'm able to engage my hyperfocus at will. Most days it lasts for 4-6 hours straight. I am able to engage it at least once a day.
This took many years to build up. It's fragile - easily prevented by distractions. But with the right set and setting, once it engages it's an incredible tool. I've accomplished things in 4-6 hours that would normally take me weeks.
I wish I could get the benefit with meds, but here we are. Maybe what I've built is the generic crappy equivalent, but it's all I've got and I'm proud I've gotten it this far.
For how I trained it, I started programming on the train ride home. I worked through low stakes programming puzzles from SICP. I found it worked best if I sprinted to the train station and ate something an hour before departure. I would open my laptop and just try to engage the hyperfocus. Many days I couldn't: people talking, train noises, being hungry, etc. Over time though I got better and better at it. Eventually I would be engaging as the laptop screen was opening. To this day ten years later opening a laptop screen engages the hyperfocus if all the rest of the requirements are met. Over a year I finished SICP, doing about one exercise a day. I then picked up another book of homework problems, then another, and another. Over a few years the skill strengthened, I found I could hold it for the drive home from the train station and keep going after.
It's harder at a desk, often I'll burn an hour or two before I can make it click. But once it does I can hold it even when getting up to go get food or going for a walk. I've gone to a cafeteria with coworkers or driven home while still in a "work trance" that picks right back up when I log in again. It's very strange being in hyperfocus on a programming puzzle while talking to someone over food. I can release it, but then I might not be able to engage it again that day so I often leave it running. Rude to my coworkers at lunch, perhaps, but I gotta make money somehow and this is how I do it.
I credit a large amount of my success to this tool. It's fantastic for just getting epic amounts of work done. I can spend hours tracing down a detailed bug, or building out a whole new module. In this trance state reading code feels as natural as reading English. Big transformations make perfect sense in ways I cannot describe outside the effect.
I've used it to write two books, build my own business, and make a living.