A mechanical watch is powered by a coiled mainspring, and a coiled spring does not push evenly from one end of its travel to the other. Fully wound it delivers more. Nearly run down it delivers less. The watch is designed to cope with that, and it copes with it imperfectly, which is where your rate comes in.
What the power reserve actually is
The reserve is how long the watch will run from fully wound with nothing topping it up. Somewhere around two days is common, and longer reserves exist.
The number is often treated as a convenience feature, which it is, but it is also describing the range of spring tension the movement has to work across. A watch at hour forty of a forty-eight hour reserve is being driven considerably more gently than the same watch at hour two.
What happens to the rate as it winds down
With less power behind it, the balance wheel swings through a smaller arc. When the arc changes, the timing changes with it, and the rate shifts.
Which way it shifts depends on the watch. This gets stated too confidently almost everywhere. Some watches run slower as the power fades and others run faster, and which one yours does is a consequence of its hairspring and how it was adjusted rather than a general law. Both are common enough that no rule of thumb is reliable, and you would have to measure your own watch to find out.
What you can rely on is the direction of the effect: the last hours of a reserve are not representative of the watch running normally. Whatever your watch does then, it is not what it does on a wrist at the start of a day.
How much wearing actually winds an automatic
An automatic winds from a weighted rotor that turns as your arm moves. Ordinary daily movement usually keeps one comfortably topped up, and that is the case it was designed for.
The situations where it does not are more common than people expect.
- A desk day. Sitting still for most of the day puts in less than you would think. A watch worn through a sedentary week can sit permanently part-wound.
- A watch taken off in the evening and put back on in the morning is losing reserve overnight and only partly replacing it during the day.
- Alternating between watches. Each one spends most of the week running down.
None of this hurts the watch. It does mean the watch is spending its life in the lower part of its power range, and therefore at whatever rate corresponds to that.
If a watch has stopped, wind it by hand before setting it rather than shaking it. Most automatics can be hand-wound at the crown, and a few turns is enough to get it running properly before the rotor takes over.
Manual watches, and the habit that helps
A manually wound watch needs winding, usually daily, and the useful habit is to do it at the same time each day rather than when it feels sluggish.
Winding every morning means the watch spends every day in roughly the same part of its power range, which makes its rate consistent. Winding it whenever you notice means the watch is sometimes near full and sometimes nearly out, and its rate will move around accordingly.
Wind gently until you feel resistance, then stop. A manual watch tells you when it is full.
Why this ruins a measurement
If you are trying to find your watch’s rate, the state of wind is one of the conditions you should be holding steady, and it is the one most easily forgotten.
A watch measured across a weekend it spent in a drawer has spent that interval winding down and then stopping. The rate that comes out describes a watch running out of power, which is not the watch you wear. It is a real measurement of something you do not care about.
The fix is straightforward. Measure across days when the watch is being worn as usual, or if it is going to sit, wind it and let it sit fully wound. Either is fine. Mixing them within one interval is what produces a figure that means nothing. How to measure watch accuracy without a timegrapher covers the other conditions worth keeping still.
Do winders help
A winder turns a watch slowly so an automatic stays wound while it is not being worn.
For the specific purpose of keeping a watch running and near full wind, it does what it says. That has real value if you are measuring, or if the watch has a calendar that is tedious to reset.
What a winder does not do is maintain the watch in any broader sense. It keeps a movement running, which means keeping it wearing, and reasonable people disagree about whether that is better or worse for a watch than sitting still. It is not a substitute for a service and it will not improve a rate. If you own one, use it because it is convenient rather than because it is doing the watch a favour.
The short version
Keep the watch wound if you want its rate to be steady, and keep it wound the same way if you want to measure that rate honestly. Where the number lands is between the watch and its adjustment. How consistently it lands there is partly up to you.