We calculate the energetics of compressible and sub-Alfvénic turbulence based on the dynamics of coherent cylindrical fluid parcels. We show that parallel and perpendicular magnetic fluctuations are generalized coordinates of the local perturbed Lagrangian of a magnetized fluid, and prove analytically that the bulk of the magnetic energy transferred to kinetic is the energy stored in the coupling between the initial and fluctuating magnetic field, ì 𝐵 0 • 𝛿 ì 𝐵/4𝜋. The analytical relations are consistent with numerical data up to second order terms.