Full Idea
The Heisenberg Uncertainty Principle says that the uncertainty in the measured energy of a system is inversely proportional to the length of time over which you observe it. (This allow near infinite energy over very short times).
Gist of Idea
Uncertainty says that energy can be very high over very short time periods
Source
Lawrence M. Krauss (A Universe from Nothing [2012], 04)
A Reaction
Apparently this brief energy is 'borrowed', and must be quickly repaid.
Book Reference
Krauss,Lawrence M.: 'A Universe from Nothing' [Simon and Schuster 2012], p.71