The tool is genuinely good. The problem is that almost every version of it is shaped like a classroom object.
A visual timer shows the remaining time as an amount rather than a number. A red wedge that shrinks, a bar that empties, an object that visibly gets smaller. You do not read it; you glance at it and you know.
That difference sounds cosmetic and is not. Reading a number is a small cognitive task, and small cognitive tasks are exactly what a distracted brain skips. Seeing that a shape is half gone costs nothing, which means you will actually do it a hundred times over a work session instead of twice.
Search for a visual timer and almost everything you find is built for children: primary colours, cartoon sound effects, a big friendly countdown meant to be seen from the back of a room. It works, but plenty of adults quietly stop using these tools because using them feels like being managed.
That reaction is worth taking seriously rather than pushing through. If a tool makes you feel like a problem being handled, you will not keep it on your desk.
A physical disc timer on the desk has one advantage no app has: it cannot show you a notification. Plenty of people run a physical timer for the room and an app for the phone. If screens are the thing pulling you away, the low-tech version is not a downgrade.
Set it shorter than the task. The point is not to finish; the point is to start, and a small visible amount of time is much easier to agree to than an open-ended block. When it ends you will usually keep going, and if you do not, you still did the part that was hard.
Do not aim for a perfect record. A timer you use twice a week for a year is worth more than a system you follow perfectly for nine days and then abandon.
Sundae turns a focus session into an ice cream that visibly melts, in real time, on screen and on your Lock Screen. Step away and it just naps: nothing withers and nothing is lost. There is a working one on the home page you can try in your browser, no install.