With the rise of digital currencies, like bitcoin, the miners of those digital currencies compete to throw raw computing power at earning those currencies, whether its their own or servers that they’ve hacked and taken over.
They don’t completely take over a server’s resources, because that would alert the owner to their presence.
Instead, they use all of the unused resources, so there is no performance loss.
When the administrator logs in and performs any tasks, the mining program hides and sleeps, until the admin logs out.
They start back up and begin once more to mine for more coins.