The extra wire is so you can check the voltage of each cell individually, or of the whole pack. 4.2 volts is a fully charged cell. 8.4 is a full charge for 2 cells. 3 volts is a discharged cell, 6 is a discharged 2 cell pack. 3.7 is the nominal voltage for one cell, 7.4 for 2 cells. So, the operating range of a cell is between 3 and 4.2 volts. You don't want to see a cell lower than 3 volts or greater than 4.2.
Don't know if I'm explaining this good, but again, 8.4 is a full charge for 2 cells while on the charger. Off the charger the voltage will be less, but a reading of around 8 volts is a chared pack.
Like you said, the voltage under load can tell a different story. A cell or pack can show full charge voltage but not be able to perform under load, the voltage may drop too low under load if the load is too great for the cell's amp rating or if the cell is just too weak to provide for the demands of the load.
So, we are only half way there because we don't know how the cells perform under load.