So, your power switch no longer "turns on" the computer. The motherboard waits until PS_RDY is on to continue booting. This is what the PS_RDY pin is for, it comes on when the power supply's internal logic determines that the power supply is "ready" and will provide stable power. Because there are some capacitors and other components in the power supply that take a moment to charge up, the voltages from the power supply's main outputs may not be stable immediately after the PSU turns on. The components of the motherboard running off of +5v SB actually turn your power supply on and off by connecting power to the PS_ON pin. The power supply will visibly turn on, with the fan spinning up. If you like to experiment, take a power supply not in a computer, plug it in, and carefully short a ground line (one of the black wires) to the PS_ON line (the green wire). These stand for "power supply on" and "power supply ready". If you look at a pinout for your power supply's motherboard (P1) connector, you'll notice two pins typically labeled PS_ON and PS_RDY. The 5v SB line importantly allows components of your computer (most importantly the BIOS and the network adapters) to keep running some simple software even when the computer is turned off. This is also why CMOS batteries aren't really as important anymore - the 5v SB is used to replace the CMOS battery whenever the power supply has mains power, so the CMOS battery is only used when you unplug the computer entirely. This is why it is important to unplug or turn off a PSU hard switch (if present) when servicing modern computers, because even when it's off you could potentially short the 5v SB and damage the motherboard. The standby power supply is a 5v line to your motherboard that is always on, even when the computer is turned off.
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