Pellet ELM pace making is one of the intensively investigated ELM mitigation techniques. Although early experiments on ASDEX Upgrade and JET revealed that frequent fueling size pellets injected from any poloidal directions into type-I ELMy H-mode plasma can trigger ELMs, the underlying physics of ELM triggering is not yet understood. The reason can be the complexity of the physical processes taking place during pellet plasma interaction. The first systematic pellet ELM triggering investigations were conducted on ASDEX Upgrade aiming at determining the pellet location at the onset of a triggered ELM defined as the onset of intensive high frequency activity seen on pick-up coil signals. The most probable location of the pellet at the MHD onset of a triggered ELM was found to be in the middle of the pedestal consistently with peeling-ballooning model of the ELM, predicting instability onset localized in the pedestal steep gradient region. Data from DIII-D also indicates that the pellet triggers an ELM before it reaches half way up the pedestal. According to detailed investigations of different kind of plasma perturbations caused by pellets on ASDEX Upgrade, and nonlinear MHD simulations of ELMs the localized high pressure pellet cloud - elongated along the magnetic field line - and the pellet caused axisymmetric plasma cooling of the whole flux surface have been recognized as candidate perturbation for trigger mechanism.