Suppose my referee friend wanted to send a swarm of bees, bats, crows, or some other small flying creatures at some players. How could he simulate something that'd evoke the right reactions?
The swarm mustn't be solid, players must reasonably be capable of walking through it or batting any solid parts of it out of the way.
The swarm must be easy to represent without too many crew - we don't want to have to recruit 100 mooks with the pitch "OK, so each one of you is a bee".
We don't want to actually kill the players. They're being attacked by the bees, so it must be reasonably possible to swing whatever it is around at them.
They must feel swarming. There must be some capability for a player to be in some way engulfed and harassed by the prop - bits should be crawling all over them as they try to swat it out of the way.
It doesn't have to be easily producible. Much as I'd love a character that can shoot bees out of their hands, we don't need that complexity right now.
It doesn't have to necessarily look much like a swarm of animals - it's fine to say "That guy over there holding the thing is representing a swarm of kestrels", players are usually capable of suspending their disbelief enough for that work work.
Suggestions so far have included circus poi, which have the right effect but could brain someone; umbrellas with the cloth taken off and bits stuck to them, which are likely too short, the players would bump into the ref holding it; and those nice jellyfish puppets, which look and feel absolutely perfect, but are generally on the end of a big aluminium pole.