How exactly does quantum uncertainty affect string mass?

[Obnoxious]

I read somewhere that the tension and therefore energy of a particular string, namely the graviton, is some enourmously large tension (don't recall exact number). Therefore, the string has a tremendous amount of energy and thus mass, however the book also says the mass is cancelled by quantum uncertainty, how is this accomplished? Wouldn't the quantum flux be upward toward a higher mass/energy just as it would be downward towards zero mass?

[Anjruu]

The quantum fluxuations are supposed to be "negative energy", and thus they add "negative mass", which detracts from the overall mass. I have no clue how something can have negative energy, however.

[Obnoxious]

I know that the energies flux at quantum levels before the measuring, but why is it the energies only flux down? Can't it also flux up and thus overload the whole thing by making some uber big string?