Assemblies, apparatus, and methods are described for designing modulation tool holder assemblies and installations for modulation-assisted machining, and, in particular, for rotating machining applications that benefit from the sinusoidal modulation motion while cutting fluids are simultaneously applied through the tool. In addition, the design of rotating modulation tool holder assemblies and systems is described. In a tool holder assembly for modulation in a rotating spindle, the electric power and/or control signals are transferred from an external source to the modulation tool holder assembly installed in the rotating machine spindle. Similarly, the high-pressure cutting fluid is transferred from a stationary source to the rotating tool holder assembly for modulation.