The main power connector contains ground and the power lines (12 v, 5v, 3.3 v., -12 v, 5 v SB) to the motherboard. It also contains a control line called "PowerOK" (or something like that) - pin 8, gray wire. This line is supposed to go to a logic HIGH (5 volts or so) after the control circuit detects that all the voltages in the PSU have risen to "in tolerance" levels. This is supposed to happen with .5 seconds after the case power switch is pressed.
The CPU needs this voltage to boot. This means that you can have all the voltages, but if you do not have this control line, nothing will happen. If this control line goes LOW during operation (short circuit or overload on one of the outputs), it forces a hardware reset, and the system will try to reboot.
And it is also why we say, "Buy good PSU's."
This is why an inadequate PSU under heavy graphics load will cause seemingly random reboot/resets.