Connecting H3 and G3 makes vid3 always report low (or zero). This does essentially overvolt the cpu, but not in a consistent manner. There are 6 vid pins (0 through 5), each capable of reporting high (1) or low (0). This makes the total voltage possibilities 64. You have to look at the Intel whitepaper document for the voltage chart (see earlier posts in this thread, or just search on Intel's site).
For example, to get 0.700 volts, all vid pins have to report high (1). If you have grounded vid3, then this makes it impossible to get 0.700 volts. Instead, when the cpu tries to ask for 0.700 volts, it ends up throwing a low vid3, but high on all other pins. This results in 0.828 volts, so the motherboard supplies the CPU with 0.828 volts. The CPU still thinks its getting 0.700 volts, so all programs that query it will see 0.700 volts.
Now, if the CPU asks for 0.828 volts (by setting all but vid3 to high), then it will actually get what it asked for, because vid3 is already grounded.
So, by grounding vid3, you ensure that whatever voltage the CPU asks for will either be equal to what it wants, or 0.128 volts higher.