The Foettinger clutch can transfer a torque only in the presence of slip. When the output speed is larger than the input speed, the pump and turbine wheel reverse their functions.
The transferred torque can be calculated according to Foettinger's law.
The transferred torque can be calculated according to Foettinger's law using the following formula:

Eq. 3-18
where: k = slip factor
Ρ = density of fluid (oil ≈ 870 kg/m3)
nP = pump speed
D = clutch diameter
The slip factor k depends on the design and the clutch slip. The figure above shows an exemplary characteristic.