Asus Crosshair V Formula/ThunderBolt - Manual (Page 48)

7.. Power.LED
The motherboard comes with a power-on switch that lights up to indicate
that the system is ON, in sleep mode, or in soft-off mode. This is a reminder
that you should shut down the system and unplug the power cable before
removing or plugging in any motherboard component. The illustration below
shows the location of the onboard power-on switch.
2-18 Chapter 2: Hardware information