Differential EXAFS (DiffEXAFS) is a novel technique for measuring atomic perturbations on a local scale that result from the modulation of a given sample property. Experiments conducted to date have revealed a sensitivity to such perturbations of the order of femtometres [1], two orders of magnitude more sensitive than is considered possible by conventional EXAFS techniques [2]. Here, the concept behind DiffEXAFS is described, and experimental factors required to detect such a signal discussed.