A PCIe riser card is a common peripheral device used by cryptocurrency miners or for any application that requires multiple GPUs to be connected to a motherboard where there may be physical limitations preventing direct attachment.
The most commonly used amongst the mining community is the PCIe 1x to 16x riser, comprising a PCB with PCIe 16x socket for a GPU to connect to, a smaller PCB with PCIe 1x connector that plugs into a PCIe slot on a motherboard and a short cable to connect the two. Typically, this is a USB3 cable. This has nothing to do with the USB protocol, it is just a conveniently high-quality cable able to transport PCIe data at the required speeds.
What that USB cable doesn't do is pass the (up to) 75W of power supplied directly via the PCIe slot on the motherboard. Instead, this power is provided externally to the PCIe 16x socket on the riser.
These PCIe risers come in a few flavours. Some provide a standard PCIe 6-pin power connector on the riser itself. The other version I've seen has a SATA power connector built in to the riser. Most will come provided with some type of adapter to allow power to be supplied via 4-pin Molex or SATA.
This is where the problem and potential fire hazard arises. SATA power maxes out at 54W1.
Even Linus Tech Tips have failed to notice or understand the danger of using SATA power. In their video "13 GPUs in ONE PC? – Mining Adventure Part 2", Linus Sebastian himself shows viewers a PCIe riser with included SATA power adapter.
You'll likely be leaving your mining rig running unattended, 24/7 for days, possibly weeks at a time. The risk of something overheating a little bit too much for a little bit too long is vastly increased.
Absolutely do not buy PCIe riser cards that integrate a SATA power connector. If you buy the ones with a 6-pin PCIe power connector, do not be tempted to use a SATA power to 6-pin adapter. Use a standard PCIe 6-pin power cable.
If your PSU has too few connectors for your intended use, you could also use a Molex to 6-pin adapter. Such an adapter needs two Molex connectors to provide enough power. If your PSU still has too few connectors to make this work, you need to buy a new (or a second) PSU. There is no other safe way of doing it!
Don't risk it. It's not worth the damage it could cause.
1.5A per pin MINIMUM. There are three 12V pins on the connector. 3 x 1.5 x 12 = 54.