(Devarim 19:5) "And one who comes with his neighbor in the forest": Just as a forest is a place that the slain and the slayer were permitted to enter, so all such places (are subsumed in the halachah) — to exclude the owner's courtyard, which the slayer had no right to enter.
"to chop wood": Abba Shaul says: Just as the chopping of wood is a (merely) "permitted" activity, so, all such activities are subsumed (in the halachah) — to exclude (a mitzvah activity, such as) a father beating his son (to chastise him), etc.