With Intel, and more recently AMD, CPUs, there is a Turbo boost mode allowing to overclock the CPU frequency of one or several cores to boost performance. This system has shown its benefits, especially for applications not written to take advantages of multicore CPUs. NVidia might be working on something similar for its Kepler chips, expected to be able to compete with current Radeon.
IT should work quite similarly than with the CPU. The system will overclock the frequency while controlling the TDP within the safety range. The goal is to provide maximal performance, without destroying the GPU or draining too much juice from the PSU.
On paper, this trick could provide serious gain of performance, especially for GPGPU applications. We will of course have to wait for the first test to be able to judge. IT will also be necessary to define how long such boost can last, if it is a matter of second, minutes, or hours. Indeed, for hardcore gamers or Pro users, if it only last few seconds, it would have much interest.