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.
However, there is a limit. You cannot use this rule for very slow shutter speeds unless you use a tripod. For example, if you are photographing a landscape scene at 20mm then I can promise that setting your shutter speed to 1/20 of a second will not advance your clarity.