How is DRS decided in F1?

the DRS (Drag Reduction system) is designated to only certain parts of a track,  most tracks will have 2 DRS zones while a few only have 1 , depends on the length of a straight piece of track,

The way in which it works is that after a certain period usually 3 laps from the start of the race DRS will be enabled by the race stewards,  and then if you are following another car and are exactly 1 second or closer behind a following car, (there are detection sensors to precisely measure these time intervals) you are allowed to initiate DRS....which basically means the rear wings are allowed to be disabled along the straight, and ONLY along that straight!,  therefore creating less resistence and less drag, which in effect gives you more power and a greater potential top speed,  as soon as the straight ends   (DRS zone)  DRS is automatically decativated untill the next   designated  DRS zone where the process is allowed to repeat providing the same conditions are met,  DRS is automatically diactivated by the race stewards as soon as it is adjudeged that the track has become to damp/wet by the stewards therefore dangerous to use DRS, because of the reduced outright grip, and visa versa

It's used when a car gets within 1 second of another car starting from lap 3. As for the locations of use, it's decided by a technical group prior to the race weekend. Normally it's done on long straights to allow for maximum opportunity for the overtake