Double, double toil and trouble; Fire burn, and cauldron bubble. (IV.i.10-11)
Remember these lines and the three witch sisters in Macbeth? These are trochees.
Shakespeare often wrote in blank verse and he rarely switches to other speech styles, but when he does, it is to communicate something out of the ordinary. When the witches make an appearance in Macbeth, the sudden shift in the speech-rhythm induces a dramatic temper to the narrative. The trochees in the verse signifies the supernatural attribute of the witches as well as the troubles awaiting Macbeth. However, trochees, in general, aren't devices meant strictly to frighten and warn audiences or readers, but here it works, especially because Shakespeare's frequently used metrical device the iambic pentameter is replaced with trochees. Besides, the words in the verse convey fear and trouble.
A trochee is not a verse type but it is a metrical foot of two syllables within a verse where a stressed syllable is followed by an unstressed syllable; unlike in the case of an iamb. In an iamb unstressed syllable is followed by a stressed syllable in a metrical foot of two syllables. And a blank verse has five iambs.
Dou ble | dou ble | toil and | trou bleThis verse, of the witches, is a trochaic tetrameter with four trochees in it.