When water churns behind a boat, the energy of motion is converted into heat through a cascade of vortices. Measuring how quickly this happens directly is difficult: you'd need to track all the vortices at once. So scientists use a simplification: they look at only one velocity component at a single point. A classic formula relates the square of this component to the overall energy dissipation rate.
It turns out this simplification doesn't always work. For average values, it's fine, but if you're interested in strong deviations from the mean, and those are what matter for understanding intermittency in turbulence, the simple formula fails. An additional term enters the calculation, one that depends on how exactly the fluid deforms. This term is described by an invariant of the strain-rate tensor — a characteristic that doesn't depend on the choice of coordinate system.
Fortunately, the error isn't catastrophic. If you assume deformations follow a Gaussian distribution (that is, random, like molecules in a gas) then the exact distribution differs from the approximate one by only a few percent. This means the simple formula can still be used in most practical applications, as long as you keep its limitations in mind.
Turbulence is one of the unsolved problems in classical physics: the equations have been known since the 19th century, but an exact solution for strong turbulence remains elusive.