When I used to power my Amiga 1200 with an ATX power supply, you could power it "in reverse" by _supplying_ +12V and +5V to the floppy disk power connector on the motherboard. But for proper operation, you also needed -12V, and that had to come from the ATX motherboard plug and go into middle hole of the Amiga's power socket.
Could any electrical engineers here explain how one gets both +12V and -12V output from a +12V input?
I assume AC is even easier, but in DC, splitting a 24V supply into 12V and -12V is trivial.
Voltage is just your distance from ground, and while we generally think ground has 0 volts, really it can have any number of volts, just whatever circuit you're running is higher. Voltage is relative to the ground potential.
To put it another way, voltage is pressure in flowing water. If you pick a river as "earth" 12V is whatever 12V is in speed faster than the flow of the river, and -12V is going the same amount slower than the river. The river is your ground potential
Could any electrical engineers here explain how one gets both +12V and -12V output from a +12V input?