Spatial and velocity distributions of repulsively desorbing products in surface aligned reactions have become a new powerful tool to study surface molecular dynamics. By an appropriate selection of the surface, elementary reactions between specified species can be well designed. Of special interest are stepped surfaces on which, depending on the coverage of admolecules, size of surface facets controls the degree of alignments. The principles of this approach are described and typical case studies are exemplified in the photo-desorption of oxygen, photo-oxidation of CO and thermal decomposition of N₂O on stepped surfaces, as well as typical application of site-sensitive desorption dynamics.