BigJETS: A hard landing can start an ELT (at least on small planes like C172's).
I think 4 G or more will trigger the ELT, but I donīt remember exactly - maybe someone else has the figures.
ELT is short of Excellent Landing Tester - old joke - I know.
We always teach our PPL-students, to check the ELT by tuning the radio on 121.5, if they have made a hard landing, thus avoiding an unneccesary SAR-action.