it actually says so on the documentation page for M5CoreS3.

But I deduced the behavior from the schematics: the reset button is connected directly to the ESP32 reset (EN) pin. At the same time it is connected to a small circuit (top middle of the schematic) which when the reset button is long pressed, slowly charges a capacitor and when it gets full it triggers the OP amp and that turns on the green LED and pulls GPIO0 low. So now when the button is released the ESP32 is reset while GPIO0 is still low and hence it restarts into download mode.

Thanks
Felix

great tutorial. Thank you for creating it.

There is an alternative way to put the M5CoreS3 into download mode I normally use: with the device already plugged into USB and running in normal mode, press and hold the reset button until the green LED lights up.

Thanks
Felix