In stock car racing, lowering the window net is an accepted signal from a driver that they are uninjured following an accident. This signal is only used if the driver is unable to return the car to pit lane or the garage under its own power. It is not a required practice, but most series consider it a "reasonable expectation". This is due to the standard safety procedures in place in most stock car series, which state that a driver must be examined at the infield medical center following an accident where the car is immobilized; lowering the window net signals to the track marshals that the driver is unharmed and does not need significant medical attention (they will still need to ride in an ambulance to the medical center).
If a driver does not lower the window net, it is seen as a signal that additional medical assistance is required.