There's an old (I believe Persian) proverb: 'a cloak is a cloak only so long as it keeps you warm'.

In the same vein, enjambment is only enjambment so long as it fulfils a practical function for the reader of your poem.

For a start, what is it? The most elementary and purely mechanistic definition is: breaking the expression of a concept across multiple lines without punctuation.

And what are its purposes? Way back in the days of the Bard, it fundamentally served to emphasise the word after the break, as in:

[...] When Duncan is asleep, –

Whereto the rather shall his day's hard journey

Soundly invite him, [...] 2

 which, 'enjambed' at the end of the second line in this fragment, brings strong focus on the word soundly, thereby stressing the likely depth of Duncan's sleep and thus the ease with which the proposed murder can be carried out. Used this way enjambment can be a very powerful tool for controlling emphasis of meaning independent of rhythm, scansion and other tools of expression.

But when is enjambment not itself? With the loosening of principles in subsequent centuries (particularly our own) the definition has widened to include: engaging or piquing the reader's interest, controlling rhythm, emphasising a word,3 or to 'create a sense of urgency, tension, or rising emotion.' 4 With the exception of controlling rhythm and emphasising a word, all of these effects are of course highly subjective, and including controlling rhythm widens the definition to include pretty much every unpunctuated line break except in unstructured free verse – which would make the definition rather pointless. In practical terms though, the primary function of enjambment is indeed emphasis. It works by introducing a mental 'pause or breathing space' 5 at the end of the line – the equivalent of what would be called a caesura if it occurred within the line itself.

Another proposed purpose from this list – creating continuity 3 is however rather more open to question. I've always thought continuity occurs naturally across unpunctuated line breaks anyway (so it's unnecessary to label each one an 'enjambment'), and this definition also seems to contradict that of providing emphasis, which by definition involves some break in continuity, however ephemeral.

But all this tinkering with definitions entirely ignores the poetic purposes for enjambment as a tool. So let's get practical. For example, here's an extract from the poem Lovework by Harry Thomas.6 Using the current definition, both line breaks are enjambments. But from the functional point of view that's not really valid.

[...] the sprinkler on full blast

Indiscriminately, spitting left and right

at mid-slope to get at the trees on both sides.

Here, whereas the first enjambment successfully stresses the significant word indiscriminately, a second notional enjambment crosses a stanza break for no apparent reason, with the relatively insignificant word at imparting no particular emphasis to the start of the second fragment of the concept. Though I suppose it might be construed as countering the loss of continuity caused by the stanza break. So I have two questions. Firstly, why a stanza break in the middle of a concept, needing an 'enjambment' to override it? And secondly, is this second enjambment functionally an enjambment anyway, or one only by virtue of an extremely loose definition?

Just like all the other tools in the poet's tool kit, enjambment must do its duty or it's just a waste of space. So here's a poem I wrote a while back that actively uses enjambment for emphasis:

Seven years on

The ancient ash tree fell

quietly: its hollow shell,

lately the home of bees,

bereft of its past might.

They, being wise,

a month before departed

knowing it had died.

Two centuries it stood

and now it rests

prone, year on year:

a lair for creeping things.

Reluctant to descend at last

into eternal night.

The first enjambment, between fell and quietly emphasises the latter (suggesting the tree subsided gently rather than crashing down), the one between departed and knowing stresses the wisdom and foresight of the bees, the third, between rests and prone ties prone to year on year more strongly than to rests, emphasising the durability of the fallen tree, and the fourth, between last and into, its unwillingness to decay to nothing. But the break between stood and and isn't functionally an enjambment as it emphasises nothing. It's only there to preserve the syllabic structure. One might suggest (tortuously) that it's there to maintain continuity, but the continuity's already there, so it's just a line break.

I might've introduced a dash or two and some functional commas (except after 'at last') that would've taken all but one of these line breaks off the list of academically defined enjambments, but to what purpose? The meaning and the form would both have stayed unchanged. But then there'd be no need to use the fancy technical term. And this exemplifies a key problem with 'poetry analysis' – an awful lot of its definitions and theories, although they sound very 'professional', have very little relevance to the real world of living poetry. In reality, enjambment is only enjambment so long as it clarifies something for the reader. Otherwise, it's just line breaks, or, at worst, jamming for its own sake.

 [1] Jamming (dance) on Wikipedia

 [2] Macbeth Act 1, Scene 7: Lady Macbeth

 [3] Quillbot blog

 [4]Oregon State University Liberal Arts

 [5] caesura: 2. In English prosody: A pause or breathing space about the middle of a metrical line, generally indicated by a pause in the sense. OED

 [6] from Lovework by Harry Thomas