That's not real. It's supposedly a mod use to modify the REPORTED hashrate to make it appear higher than what the EFFECTIVE actually is. The video you linked is all a scam for $400+

See https://bitcointalk.org/index.php?topic=2145776.0

So now, back to your question, why is it detrimental to mine at 30% less power when the effective hashrate is the same as it would be on 100% power? And I mine ETH on NV cards because 1) it's more efficient. You are getting 30Mh/s on 100-110W with NV vs. 29-31Mh/s whatever the wattage is for the rx 470/480/580s which I believe was around 130s-140s when I looked into it a while ago. On top of that, you don't have to go through all of the BIOS mods with NV as you would for the Rx cards and risk the chance of bricking.