The right movement.

An automatic watch gets its energy from a rotor: a metal half-disc that turns freely inside the case. Your wrist movements turn it, winding the mainspring, which then supplies energy to the movement. No battery, no power connection.

When the watch remains motionless, the spring relaxes and the watch stops. Depending on the calibres, the power reserve ranges from 38 hours to several days.

A winder reproduces wrist movement in programmed cycles. The watch keeps running, on time, with its complications up to date. On a watch with an annual calendar, moon phase or perpetual calendar, this avoids a lengthy reset each time you wear it.

The main setting is measured in turns per day, often abbreviated TPD. Most calibres need between 650 and 1,000 turns per day. Too low a setting leaves the watch insufficiently wound; too high a setting does not damage the movement, as modern slipping clutches limit spring tension, but runs the motor unnecessarily.

The exact value depends on your calibre. Consult the manufacturer's documentation or ask your watchmaker.

There are three settings: clockwise, counterclockwise and bidirectional. Most movements wind in both directions and work well in bidirectional mode, which distributes wear. Some calibres wind in only one direction; for these, a dedicated direction is more effective.

Quartz watch: no use, because it runs on a battery. Hand-wound watch: no use, because it is wound using the crown. One watch worn every day: no use, because your wrist does the work.

This question comes up often. A running movement wears, which is true. But a movement left stopped for a long time can develop thickened, unevenly distributed oils. For watches worn in rotation, a winder is a reasonable compromise. For a watch you will not wear for several years, leave it in a box.