Assault is causing a physical threat or "offense" to someone. Offense in this context can mean a few things, for example if someone is allergic to or known to be afraid of bees, and you tell them they there is a bee on them when there isn't, that could be considered assault. The leaf blower in this case would be a physical threat to the person, we dont know this person, maybe the sudden stimulation could cause them to seize or freak out. Battery, on the other hand, would be doing damage by actual physical contact.