Drag Reduction System was introduced in 2011. to help drivers overtake by reducing the angle of attack of rear wing flap, which is strictly controlled by the rules.
In poor weather conditions or yellow flags conditions race director can disallow the DRS usage. During practice and qualifying, drivers can use the system within DRS zones (in 2011. and 2012. they could use it wherever they wanted), but if system is disabled at any point during Q1, Q2 or Q3 it remains disabled for the rest of that period.
During the race drivers are not allowed to use DRS in first two laps after the race start of a safety car restart, but can immediately use it after Virtual Safety Car (VSC) period.
In the race, drivers may only activate DRS when they are within one second of the car in front at the DRS detection point, which is visible on driver’s steering wheel.
DRS is automatically disabled when drivers use their brakes or press the activation button again to set the rear wing in its original position.
FIA F1 2017. sporting regulations can be downloaded here.