Essentially they're recreating a stimulus in the brain that occurred when the initial memory was created. I don't know if this means generally stimulating better recall, but it has interesting implications for students. If you study with a particular tonal pattern in the background, then stimulating a matching theta wave as was generated by that tonal pattern might also stimulate better recall in that instance.
Perhaps memory was originally developed for same-environment tasks, e.g. remembering complex routes through a forest at a certain time of day in a certain season.