A watch winder is a device the holds your automatic watch and rotates it to keep the mainspring wound. Mechanical movements, automatic and hand-wound, both benefit from having their mainsprings work constantly.
Why? Because moving watch parts keep the tiny amounts of lubrication around the pivot points from "gumming up." I do believe that is the technical term for it (not really). Gummy lubricants increase resistance which reduces timekeeping accuracy.
Parts of a winder
Winders, like watches, come in a dizzying number of variations. Unlike the timepieces they cradle, there are fewer components. Let's take a look at these.
The case - This is the "box" that houses the motor, the head, and the controls. It can be utilitarian (I have seen skeletonized models) or an artfully crafted piece of furniture. The case may include a jewelry or valet tray. It may have a cover over the heads or not. It may be locking. The size of the case depends upon the number of heads. A travel winder can fit in a carry-on while others can fill an entire wall.
The heads - These are the cups and pillows that keep the watch secure while the winder is operating. The cup should be lined with soft material to protect the finish of your timepiece. The pillow should also be covered in similar materials and have sturdy foam filling. Once set into the cup, the watch should not move as the motor rotates.
Note: Heads come in different diameters. If you have some larger watches (50 mm+), be sure the winder head can accommodate it.
The motor(s) - It is common for multi-head winders to have a drive motor for each cup. This allows you to turn off any motor not in use. Outside of travel models, winders typically run on standard household voltage. Place the winder near an outlet, plug it in, insert the watch, and turn it on.
The controls - The most basic winder provides switches that allow the motor to turn clockwise or counterclockwise (bi-directional). This is important because some movements should only be wound in one direction. Check your owner’s manual and watch specs for details.
The other consideration is this, each movement has a recommended number of rotations for optimized winding. Some winders can be programmed to rotate a specific number of turns. Others also allow you to alternate the rotation back and forth. The nature of your movement will determine how elaborate your controls should be...or, your desire as a gadget freak to have cool new toys may rule. It’s a no-judgement decision.