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In the official Apple support guide to reset the SMC of a Mac with the T2 chip, it says:

Before resetting the SMC, try these steps:

  1. Shut down your Mac.
  2. Press and hold the power button for 10 seconds, then release the button.
  3. Wait a few seconds, then press the power button to turn on your Mac.

Reset the SMC on computers that have the T2 chip

What does step 2 do?

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    It might be a forced capacitor power-drain for a machine with a permanently fixed power source… but that's just a guess. There's a similar procedure for firmware updates on old Mac Pros, the 'long sit & wait whilst holding' time tells it to look for the update.
    – Tetsujin
    Jul 28, 2021 at 15:05

2 Answers 2

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Step 2 is a forced shutdown of the machine. Internally it likely causes the system to clean out a few caches, run some verification checks, and the like, because the system assumes a forced shutdown means the user experienced a problem. In effect, these three steps ask you to do a normal shutdown (so you don't lose open data), start the machine and immediately perform a forced shutdown, and then restart it to see if that fixed the problem.

It's a bit more high-tech than slapping the side of the computer to see if that sorts it out, but kind of in the same spirit.

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Step 2 simply tells the hardware that you want the SMC chip cache to be reset. It in itself likely doesn't do anything other than differentiate to the hardware your intention. This way, there is no confusion between a 'normal' power button press and one that requests resets.

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    i don't think that's quite right. if press and hold the power button for 10 secs resets the SMC chip cache, why are there further instructions (press and hold ⌃+⌥+⇧, keep holding for 7s) down the list?
    – hongsy
    Jul 28, 2021 at 14:31
  • There are other 'resets' and requests of the hardware: PRAM reset, safe mode, target disk mode, Startup Manager, Startup from recovery, etc. All different because that is how you are communicating your wishes to the hardware.
    – cmason
    Jul 28, 2021 at 15:21
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    No other reset has this instruction first, though. The others all just start from simply powered off or even a straight reboot, including the actual SMC reset. So it has to do something 'other'. It's not actually part of the reset.
    – Tetsujin
    Jul 28, 2021 at 15:56
  • @cmason so if there are other types of resets, exactly what reset is 1) press + hold power button for 10s and 2) what reset is press + hold ⌃+⌥+⇧ for 7s?
    – hongsy
    Jul 29, 2021 at 4:02
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    1) is forced shutdown… yet at the point you are told to do this, the machine is already shut down. The SMC reset instructions are separate to this first set of instructions & are for if this doesn't work.
    – Tetsujin
    Jul 29, 2021 at 13:03

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