A theory is presented of a new heat propagation mechanism, thermal hydraulic quenchback (THQB), in large scale superconducting magnets. The underlying physics of THQB is discussed and an analytic solution for the quench propagation velocity is presented. This solution represents the first such result for THQB, and is shown to be in excellent agreement with the full numerical simulations of the governing mass, momentum, and energy conservation equations for the compressible flow of the coolant in the conduit. The THQB propagation velocity is observed to be as much as an order of magnitude greater than the helium coolant velocity. This is in direct contrast to a standard quench where the two velocities are always essentially equal.