How exactly does getting 7V by connecting both terminals to the 12V and 5V sources work?
Yes 12V - 5V= 7V.
The catch is 12V would source current but the 5V is being pulled up and you would need a 5V load on the MOBO to prevent overvoltage shutdown from any load sinking current more than what might being sourced up to 12V difference load to use that 7V.
The Forward converters pulse charge up to 5V via tightly coupled transformers and rely on load to bring down. (AFAIK)



simulate this circuit &ndash; Schematic created using CircuitLab
By the way, the same is true for Buck converters and LDO's, whereas a &quot;push-pull regulator&quot; can source or sink current.