You can easily verify yourself, as I did with a multimeter
that is exactly what i did - of course there is no 3.3V connected.
actually there is no logic connecting it as riser makes it's own 3.3V from 5V input...
Your confirmation that they share a ground and yet are electrically isolated shows you are clueless and I would DEFINITELY NOT listen to your advice.
sharing ground does not mean that devices can not be electrically isolated.
your TV and microwave owen share ground yet they are electrically isolated.
you are uneducated idiot no wonder you gather your "knowledge" from other idiots on youtube lol
I proved and anyone that knows how to use a multimeter can prove as well the riser and motherboard
ARE NOT electrically isolated and therfore as recommended by a PSU manufacturer, and someone with over 15,000 people following him on YouTube for his experience with mining rigs, you should never mix power supplies to devices connected to the motherboard. The post below from someone with an electronics backgorund also explains why the same PSU that powers the motherboard MUST also power the risers.
https://bitcointalk.org/index.php?topic=1843586.msg18399752#msg18399752Which is a lot more reason to follow that advice than from someone too stupid to use a multimeter and understand
again, that risers ARE NOT electrically isolated from the motherboard.