Here's why:
* Participial phrase: A participial phrase is a group of words that includes a participle (a verb form ending in -ing or -ed) and its modifiers.
* Participle: "Switching" is the participle in this phrase. It is a present participle formed from the verb "switch".
* Modifier: "Their tails" is the modifier of the participle "switching". It tells us whose tails are being switched.
The entire phrase "Switching their tails" acts as an adjective modifying the noun "horses".