Here's what it means:
* Speed: Flyswatters are known for their swift movements, being able to quickly strike down flies.
* Agility: They are also flexible and able to change directions rapidly, allowing them to hit the target even if the fly is moving erratically.
Therefore, the simile implies that whatever is being compared to a flyswatter is:
* Very fast
* Quick to react
* Agile and nimble
Example: "The boxer was as quick as a flyswatter, dodging every punch his opponent threw."
In this example, the simile emphasizes the boxer's incredible reflexes and ability to move quickly and unexpectedly.