After " before" is the best place to put participle phrase " synchronizing their watches."
Explanation:
A participle is a form of verb that functions as an adjective to modify nouns and pronouns. A participle can be of two types present participle and past participle.
A participle phrase includes a participle along with a compliments as well as modifiers of that participle. A participle phrase is actually an adverb but it looks like a verb. For example Walking on the sea beach where added information has been provided along with the noun regarding its doing as well as its look.
