You must first understand the kinematics of the rotating system. That includes the mathematics of rotating systems like rotational matrices and rotating co-ordinate systems. Later you can start to use rotational dynamics. First you consider the torques on fixed systems and later the real and imaginary forces on the rotational system. Finally you consider the dynamics of a free rotating system.
Physical action is the integral of the generalized momentum along generalized distances.
Therefore it has the dimensions of the product of energy and time. What is important is that you can express it as the integral of the LaGrangian function with respect to time. The LaGrangian function is the difference between the kinetic energy and the potential energy. Understandably, you can conjecture that the LaGrangian function is the foundation of LaGrangian dynamics. From the Principle of Hamilton, you can derive LaGrang's Equation.