The timer is one part of it, and indeed, a simple kitchen timer is all you really need. But most of the Technique is about tracking your work through backlogs, estimating it, and (most importantly) analyzing it retrospectively: https://archive.org/details/ThePomodoroTechnique/ThePomodoro... It's like a personal mini-Scrum with planning and retrospectives. Flowkeeper mainly automates that "paperwork" part.

Also, Pomodoro Technique requires discipline, and most of the simple timers don't help you much. For example, if you just cancel a pomodoro (or a short break) -- nothing happens, while in Flowkeeper it is more visible, you get an interruption recorded. There's a bunch of small things like that, which motivate the user to do Pomodoro _right_. When you do it right, it is much more efficient.

Just to be clear -- a kitchen timer and a sheet of paper is enough. I just wanted to do the _exact_ same thing for my desktop.