H-2 diffraction from the Ru(001) surface has been measured for incident energies E-i = 78-150 meV and incident angles Theta(i) = 22.1-64.1 degrees. In-plane and out-of-plane angular distributions were measured for incidence along [110] and [100] directions. Out-of-plane diffraction channels were found to be predominant for the explored experimental conditions regardless of the incidence direction. An analysis of diffraction intensities reveals that diffraction out of the scattering plane enhanced for high incidence angles. Diffractive transitions with wavevector change in the surface plane and transversal to the incidence direction Delta K-perpendicular to were observed to be favored among the out-of-plane diffractive transitions. These features could be reproduced by model calculations of diffraction intensities performed using a three-dimensional soft potential. This suggests that a kinematic effect is responsible for the large out-of-plane intensities observed in experiment, more than any other features of the six-dimensional H-2-surface interaction potential.