The explicit unsteady approach is the proper choice if the unsteady time scales are of the order of the acoustic processes (for example, shock front tracking).
The explicit unsteady approach is effectively the same as the explicit integration scheme available for steady-state simulations, but using a constant time-step for all cells in the domain (rather than local time-stepping). In this case the size of the time-step is determined automatically by the solver such that one value satisfies the Courant condition at all points (that is, the minimum allowable time-step is used everywhere). Thus each iteration becomes a time-accurate advancement of the solution. In this case the Courant number should be 1 or less, resulting in a physical time-step that varies from one iteration to the next as the flow field changes. Furthermore, preconditioning of the governing equations is omitted, thus making the explicit unsteady approach unsuited for incompressible flow simulations.