why do molex to 6 pin power adaptors only have 5 pins?

[Carl Patrick]

i have a power adapter that i used to use but now dont need but i looked at it and i saw that it only had 5 pins on the "6 pin" part even though both molex cables combined have 6 pins. the adapter works just fine but i dont know why they have 1 cables from the molex going into 1 pin on the "6 pin" end

 

[ko888]

Pin 5, on the 6-pin PCI-E supplementary power connector, is the Sense pin. It can be left open (i.e. unconnected) or connected to ground.