Fluid dynamics in a turbine or torque multiplier with the stater. Turbines uses the fluid to spin up and they usually have a lockup clutch for cruising. Torque converters are usually smoother but don’t deliver the full power right away, which is why some prefer a manual. Double clutch trannies are another beast which I love on my vw. I would assume the cvt takes the place of the clutch in the torque converter?