An amphibrachic foot (known as an amphibrach) has a short syllable followed by a long syllable followed by a short syllable (SLS or U/U).
Pentameter is five feet per line.
Amphibrachic pentameter is obviously not the poet’s favourite (we were unable to find any examples), but the following two lines illustrate how it could be used:
The figure was walking between us; we both saw it stumble.
It fell, then a crowd gathered round and soon started to mumble.
Listen to those lines in your mind’s ear, or read them out loud, and you will probably realise why poets don’t normally use pure amphibrachic pentameter. Rather monotonous, isn’t it?
A closer look at the feet in these lines
U = short syllable; / = long syllable; | = division between feet