Flat beams feature unequal emittances in the horizontal and vertical phase space. Such beams were created successfully in electron machines by applying effective stand-alone solenoid fringe fields in the electron gun. Extension of this method to ion beams was proposed conceptually. This contribution is on the decoupling capabilities of an ion beam emittance transfer line. The proposed beam line provides a single-knob-tool to partition the horizontal and vertical rms emittances, while keeping the product of the two emittances constant as well as the transverse rms Twiss parameters (αx,y and βx,y ) in both planes. It is shown that this single knob is the solenoid field strength, and now we fully understand the decoupling features.