What are "sense" pins in 8-pin PCI Express power plug?
Sense pins are connected to ground at power supply or adapter cable. This allows the PCIE card to detect if a supply cable is connected or not, and whether a 6-pin supply is connected to 8-pin socket to indicate less power is available. It is not used by the power supply for "remote voltage sensing" to compensate for voltage drop over the wiring.
The 6 pin PCIe power connector comes in a 6.25A (75W) version and a 12.5A (150W) version. The 6.25A version only requires that two 12V power connections be present, and only two ground connections need to carry current. The 3rd middle ground connection can be used to sense that the connection is plugged in. The 12.5A version uses 3 12V power wires and 3 ground wires to carry current. An additional two groundable sense connections can be added to the side to tell the PCIe device that a 12.5A compliant power connector has been inserted. Many power supplies that are 12.5A compliant have an additional 2 pin ground connector on the side of the 6 pin connector that can be used to make it to an 8 pin connector.
If a 12.5A 6-pin connector on an older power supply is connected to a newer 8-pin PCIe device, the PCIe device should not turn on because it doesn't know if a 12.5A compliant connection has been used. If all 6 wires are present on the 6 pin connector and the power supply can handle the current, then it is safe to get an adapter that will convert the 6 pin connector in to an 8 pin connector simply by grounding the two additional connections on the 8 pin connector.
The 2 sense connections on a 8 pin PCIe board could be permanently connected to the 6 pin ground connections with a little bit of solder and wire. This would convert it in to a 6 pin connection. Just be careful to not use any 6 pin PCIe power connectors that only have 2 12V wires, and thin wires that are smaller than AWG 20. If one fails to connect, all the current will go through the remaining wire and it could melt. If this is bad advice, then online sellers that sell Molex to 8 pin PCIe adapters should stop selling those first. That could easily end up runing everything through one wire.
These are used by the power supply to compensate for voltage drop in the cables and connector. These are low-current returns to the power supply. The PS senses the actual voltages on the PCI board and adjusts the outputs so that the yellow pins are as close to 12 volts as possible and the black pins are as close to zero volts as possible.
For example, say that the +12v has a 0.25v drop because of oxidation on the connector pin producing resistance. The green sense pin is not affected by oxidation due to the very low current flow so the PS senses this and raises the +12v output by 0.25v so that the board receives 12.0v.
Similarly, the blue sense pin allows sensing voltage drop on the black wires, allowing the PS to drive the black wires slightly negative to compensate.
Looking at the location of the sense pins, I believe @justme is correct. This allows the PCI card to detect what size connector is plugged in and so know how much power can be drawn safely.