Risk Reduction is trying to make sure bad things happen as rarely as possible. It's anti-lock brakes and vaccinations and irons that turn off by themselves and all sorts of things that we think of as safety modifications in our life.
Harm Mitigation is what we do so that when bad things do happen, they are less catastrophic. Building fire sprinklers and seatbelts and needle exchanges are all about making the consequences of something bad less terrible.
This talk is focused on understanding where we can prevent problems and where we can just make them less bad.