An experimental study is performed to investigate the underlying physics of ELM triggering by imposing local perturbations at the plasma edge. Deuterium is injected during type-I ELMy H-mode phases by small solid pellets or as a supersonic gas jet. In both cases the repetition rate is small compared with the intrinsic ELM frequency, aiming for a small perturbation of the intrinsic ELM cycle. Active triggering of ELMs requires a density perturbation in the gradient region about 1 cm inside the separatrix (measured at horizontal mid-plane), a condition that is achieved by pellets of any size and speed but not by even the strongest available gas jets. The pellet-induced triggering of ELMs always occurs when pellets reach the H-mode barrier region. A density perturbation produced by a gas jet remains localized at the separatrix, and even if larger by a factor of at least 100, is not sufficient to trigger an ELM. No significant dependence of the ELM size on the ELM onset time was found, measured from the onset of the previous ELM.