When fluid is moving through the sensor's tubes, Coriolis forces are induced in both the inlet and outlet legs of both flow tubes. These forces cause the flow tubes to twist in opposition to each other.
The mass flow moving through the inlet legs of the flow tubes generate a Coriolis force that resists the vibration of the flow tubes. As the mass flow moves through the outlet legs, the Coriolis force adds to the vibration of the flow tubes. It is the opposite direction of the Coriolis force between the inlet and outlet legs that result in the twisting motion that is used to measure mass flow rate.