Russell Barkley's framing, which is widely used in clinical practice, is that ADHD is better described as a performance problem than a knowledge problem. Knowing what to do and when is rarely the gap. Doing it at the moment it needs doing is. His practical conclusion is that useful interventions put the missing function outside the person and place it at the point of performance. Visible clocks and timers count. So do written cues sitting where the work happens, rather than structure you have to remember to apply.
Difficulty judging elapsed time, often called time blindness, is one of the most commonly described features of adult ADHD, and it is why a countdown you can see does more than an intention to work for a while. Twenty-five minutes stops being an abstract quantity and becomes a shrinking number on the screen. The ChronoClock timer keeps the remaining time in the browser tab title and can fire a browser notification at the end of an interval, which matters if the timer window ends up buried behind something else.
The second useful property is a lower activation cost. The hardest moment is the one before starting, and the size of the commitment is what makes it hard. "Work on the report" has no visible end. "Fifteen minutes on the report" does, and a bounded commitment is easier to agree to. This is also why shortening the interval is usually the right move when the method starts failing, not lengthening it.
Third, breaks stop being a negotiation. Without a schedule, breaks either never happen or turn into the rest of the afternoon. A timed break is permission with an end on it, and it legitimizes moving, which for a lot of people is not optional.