Flip the iamb and you get the trochee: stress first, slack second. The result is a falling rhythm that feels urgent, chant-like, or magical - which is why Blake's 'The Tyger', Longfellow's 'Song of Hiawatha', and most of the spells and incantations in English drama are trochaic.
Because English sentences usually end on a strong beat, pure trochaic lines often drop their final unstressed syllable - a 'catalectic' line. Blake's famous line above is catalectic trochaic tetrameter: four trochees minus the last slack syllable.
Trochees also appear inside iambic poems as substitutions. An opening trochee ('Now is the winter of our discontent') adds dramatic punch before the iambic pulse resumes. Automatic scansion tools must therefore distinguish a genuinely trochaic line from an iambic line with one inversion.
Paste a suspected-trochaic passage into the Poetry Meter Checker or run a whole-poem audit with the Scansion Checker to see where falling feet begin and end.