Facial twitch: I believe these twitches are similar to the twitches people get in their eyelids. Usually it goes away within 6 weeks or so. There are other causes such as caffeine, stress, alcohol, electrolyte imbalances and excessive fatigue. Feel better.
A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by one stop. Remember that fractions get larger as the number on the bottom gets smaller.
Photographers set their exposure using a combination of shutter speeds and f/stops to get the correct amount of light on the sensor (or film). The shutter speed regulates how long the sensor is exposed to light coming through the lens. The f/stop regulates how much light is allowed through the lens by varying the size of the hole the light comes through. For any given film speed (ISO) and lighting combination there is one correct amount of light to properly expose the image.