What the Kohi test was
Kohi was a Minecraft HCF server whose community needed a shared way to compare click rates, and the ten-second CPS test it hosted became the reference nearly everyone in PvP quoted. The server is long gone; the format outlived it, which is why a ten-second click test is still called a Kohi test.
There was never anything special about the measurement itself — it counted clicks for ten seconds and divided by ten. What made it durable was that everyone agreed on the same ten seconds.