This is the Xpert Electronic Switch.
This electronic switch is more reliable than a standard mechanical switch, and is compatible with 6.0V NiMH receiver packs, as well as 7.4V LiPo.The switch uses a series of LED sequences to indicate the type of battery and the voltage of the battery selected by the user. When the Blue L1 light is solid it is in LiPo mode, and when the light is flashing it is in NiMH mode. When the battery voltage gets low (5.3V NiMH/6.8V LiPo), the Red L2 light will flash, and when the battery reaches the low voltage point (4.8V NiMH/6.4V LiPo), the red LED will stay lit constantly so you know its time to recharge/replace the battery.