If you have a watch with a date, there are three positions on your crown. When you unscrew it, and it springs out, this is where you can manually wind it. The second position is the date, and all the way out changes the time.
When you manually wind it, just give it about 30 turns or so is fine but you can't over wind it so don't worry. (Unless its very very old model).
The reason you may want to manually wind it is this:
The interworking of the movement has what's called a mainspring, it looks like a little coil if you have ever seen a watch with a window, and looks like it springs open and closed very fast. This is the source of the energy that powers the gears. The spring must be wound up in order to release the energy. In an automatic movement there is what's called a rotar, you can think of it like a tiny pendulum, similar to a old grandfather clock. It looks like a half moon and secured in the center, as you move your wrist the rotar swings back and forth, automatically Winding the spring.
A fully wound watch will run for approximately 48 hours or so. If you wear it all the time, the reserve is pretty consistent. If you take it off and it completely stops, you have depleted any reserve.
Now say you pick it back up, by the time you handle it and set the date the rotar is turning and the second hand starts moving. But you have no reserve and the watch is constantly trying to catch up.
Honestly most people never know this or do it. Rolex says to go ahead and do it once a month when you flip the date (or every other month). I do not have a date, I wear mine every day of the year almost. Unless it necessary to change the time I open my crown as little as possible. Excessive screwing and unscrewing can wear the threads out.
Never never leave your crown open. Check it twice!
