If the driver is travelling at more than around 30mph, faces an emergency and hits the brakes so hard that pressure exceeds 75 per cent of its maximum level, then the brake lights will start flashing to warn anybody travelling behind. Once Vito has screeched to a halt the hazard warning lights will come on automatically.
If the emergency isn't as serious as the driver thought it was and he reduces pedal pressure significantly during the braking period, then the brake lights will cease to flash and will revert to standard mode.