Powertweak tweaks the internal registers of your chipset and processor. This is a secure operation, since these registers have been designed to be programmed in order to make the hardware perform best.
More precisely, Powertweak configures the way the processor and the chipset talk. Most modern components can be configured in order to enable higher troughput modes.
The chipset is the main component of your mainboard. It manages the data flows between the PCI bus, the AGP bus, the memory and the processor !
The processor is the heart of your system, and it's extremely important to configure it correctly.
So Powertweak can increase the speed of your hardware by up to 30%, depending on the quality of your drivers, your BIOS, etc.
All this is done securely : you don't have to specify your chipset or processor, Powertweak finds what's installed on your computer, and tries to tweak it accordingly.