PWR_EN0 is an inverted power-enable signal (e.g. "active" is "0"). 
This signal serves the same purpose but has the opposite state of PWR_EN for which the "active" state is "1".

In essence this will permit more options for selecting the power-controlling circuits (as some require a "1" for "on" state and others require a "0" for the same).

For PWR_EN0 the two states are:
Output "0" - power is on (bitfury chips are powered)
Output "1" or Input - power is off (butfury chips are not powered)