A nice feature of this PSU is the soft ON/OFF, our vintage equipment is treated gently avoiding abrupt current changes, which might damage it. So you should leave you power button always ON on your C64 and operate from the power switch of the PSU.
5V measuring point:
Soft ON from PSU switch (5ms/div):
Soft OFF from PSU switch (20ms/div):
ON from C64 power switch (5ms/div):
OFF from C64 power switch (20ms/div):
