It has not yet been released by GW on how they control the T'au. From the Warhammer Community Post:
Undocumented Methods of Control
Ethereals possess an absolute monopoly on the truth in the T’au Empire, allowing them to orchestrate grand campaigns of propaganda via their advanced communication technology. Good press is a powerful tool in their arsenal, but do they release the whole truth?
Imperial biologians have proposed several ways the Ethereals might exercise control, from the popular theories of veiled psychic domination to pheromonal manipulation. Some outliers have even proposed mass hypnosis or invisible organic nanites to control their subjects. Either way, the extent of their sway over the empire is mythical and utterly alien to the Imperium – who secretly wish they were even half as good at it.
Found while investigating another question. A statement from Gav Thorpe about the T'au:
The Shishell had psykers ruling over them, whereas the T’au most definitely do not.
From Gav Thorpe's websites entry on the Origins of the Tau
Specially interesting, since he created the Tau
Gavin Thorpe began developing what eventually became the Tau in the early 1990s.
From the Wikipedia article on the T'au Empire
So, we don't know what it is but we know what it isn't.