The Blue wire on the Smartphone Controlled Motorcycle Box is for the automatic brake lights. The automatic brake light feature illuminates all installed LEDs in red mirroring the motorcycle’s brake lights, safely signaling other motorists on the road. Pull the brake handle and the brake light feature automatically illuminates red LEDs throughout the bike. This feature can be used whether the control box is powered on or off. There are 2 different types of brake light features available in the app: brake light and brake pulse. Brake light illuminates all LEDs in solid red; brake pulse automatically strobes red LEDs into a solid color mode. To use this function, install the blue wire from the control box into the motorcycle’s brake signal wires.