Apparently, squirrels flick their tails to distract predators from their body. They are saying "I see you predator! If you attack me, go for my fluffy tail that has no critical organs in it!"
Squirrels flash their tails for a lot or reasons, including trying to attract mates, as well as distract predators, but they also use their tails to communicate with each other. Tree squirrels typically can communicate with each other via a kind of semaphore sent by flashing their tails and most of their tail flashing is squirrels silently communicating with each other.
But flicking would seem to attract attention that being still wouldn't.
Check out stoats hunting rabbits. Their strange antics put rabbits into a stupor, letting them eventually approach with little resistance.