I was going to post Interitus
was mistaken when he made that claim, because surely a recent board like the MSI K8N Neo4 Platinum SLI could not demand a -5V line that has been dropped from the power supply standards since version 1.3 - which dates back a few years.
But, then I decided to look around a bit since I was considering this board for a friend... and found stuff like this
(MSI forum link). It appears that this particular MSI board requires a PSU with a -5V line
, i.e., a PSU that is neither ATX1.3 or ATX2.0 but something completely different. Besides the -5V line, the power supply should have a 24pin connector and supply in excess of 26A on the 12V line. That's one "which one to buy" question the recommended lists won't answer I think. Apparantly there are a few PSUs out there that can actually meet these requirements (see the linked forum for them). Their noise-level will definitely be anything but that of the Phantom though...
To answer your question, the -5V line (or minus five Volts line) is basically a different wire coming from the PSU that supplies another voltage to the motherboard, alongside the 3.3V, 5V and 12V already there. It used to be required (I think by old ISA cards) but it has not been needed for a long time... that is, until MSI decided to dig it up as a requirement. Truth be told, this is due to the on-board Creative sound chip, but I still think this is very odd. This is one of the most obscure incompatibility issues I have run across in the last year or so... and not one I'm about to get involved in.