Swarms are not threatening, but be prepared for an adrenaline boost anyway!
The bees have gorged themselves on honey for the trip, they are homeless, have no honey or brood to defend and are focused on their mission to find a new permanent location.
Placing a box below the swarm and giving the branch a good smack removes all the bees in one action.
This looked like a good cover for a swarm box, but didn't work. In order to leave the colony, the swarming queen stops egg production in order to reduce her body weight for flight. She is then small enough to fit through this bars in a queen excluder. The swarm left this box and started their journey over.