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.
Then the detector automatically shuts the light off unless it continues to sense movement. A photocell deactivates the light during daylight hours. Most motion detectors have a semicircular field of view of up to 240 degrees and a distance range, adjustable on most detectors, that extends to 70 ft. or more.