T S Eliot distinguished between free verse which is created by making variations on a metrical pattern or norm (verse libre) and verse which is has no such relation (verse liberé).
Free Verse as Variation
1 Let us go, then, you and I, /x/x/x/
2 When the evening is spread out against the sky xx/xx/xx/x/
3 Like a patient etherised upon a table; xx/x/xxx/x/x
4 Let us go, through certain half-deserted streets, /x/x/x/x/x/
5 The muttering retreats x/x/x/
6 Of restless nights in one-night cheap hotels x/x/x/x/x/
7 And sawdust restaurants with oyster-shells x/x/x/x/x/
(The Love Song of J Alfred Prufrock)
Eliot has the traditional iambic pentameter in mind, but begins off-centre, as it were.
is not iambic, but similar in having an alternation of stress and unstress but starting with the stress (trochaic). And it’s a tetrameter (4 stresses) not a pentameter (5 stresses)
matches L1 in having four stresses, but is not iambic. It’s mainly anapaestic and nearer to the pentameter in that there are 11 syllables.
also has four stresses and starts with an anapaest, but then has a few trochees.
You could possibly scan ‘etherised’ as /x
Suddenly and decisively (with the decision idea ‘let us…’) gets completely regular, every foot a trochee, and for the first time we have a pentameter.
continues with regularity, if you accept my slightly doubtful scansion, where I’ve scanned ‘muttering’ as /x/ which is influenced by the idea of iamb which emerges in this line. A more strictly naturalistic analysis of ‘muttering’ would have to be /xx.
L6 now comes out with the – as it were – ‘held back’ norm, the iambic pentameter.
L7 reinforces this, although my scansion of ‘restaurants’ (/x/)takes up the poetic license of ‘promoting’ some syllables so as to create regularity. A more naturalist analysis would be /xx or even /x
This passage, then, begins with variations and finds its way to the ‘theme’ metre after a series of lines which ‘hint’ at it by each have something in common with the iambic pentameter.
One way of writing this kind of verse would be to draft something in the stricter form and then in redrafting work some improvised variations on it.
Free Verse without a base form
1 The ploughed chalk sweeping x///x 3
2 and shelving is a shore x/xxxx/ 2
3 from which the tide has just gone out. xxx/x/x/ 3
4 Fine, black blades /// 3
5 of trees stand against depths x//xx/ 3
6 which the sun fills, xx// 2
7 white and cold. /x/ 2
8 A big hare sits with ears up x///x// 5
9 on the rim of the world. xx/xx/ 2
10 Larks rise singing from the ocean bed. ///xxx/x/ 5
The analysis on the right shows clearly enough that Hooker’s not aiming at regularity, either in the number of stresses per line or in the syllable patterning. It’s definitely not ‘iambic’.
The poem is shaped rhythmically by the rhythm the sentences generate. Each stanza is a sentence, and they are parallel in meaning in that each makes an existential kind of statement about the landscape
is - stand - sits - rise
The first two stanzas/sentences are parallel in rhythm to some extent. Both have the quite striking /// pattern, three stresses together in a row, in their opening lines. Putting stresses together slows down the tempo of the line and enhances the kind of stasis which often interests Hooker, who is very much into place and landscape which are fundamentally still things. The same /// recurs in line 8, which beautifully captures the stillness of the hare set against the mystical ‘rim of the world’. The verbs above reflect this except for ‘rise’ which is the one motion verb in the poem.
Stanzas 1 and 2 also both end with a /x/ pattern, as does the last line of the poem. Stanzas 1 and 2 both also have ‘which’ clauses in them.
lines 9 and 10 both have place clauses - ‘on the rim of the world’ and ‘from the ocean bed’, which echo each other rhythmically xx/xx/ and xx/x/. This parallelism of sound enhances the mysterious contrast between the momentary fixed hare and the larks seeming to rise from the bottom of the sea. 
A more basic overall pattern can be seen if we look just at the number of stresses, all the lines having either 3 or 2, a two having 5 (which have internal pauses breaking them into 2 and 3 stress ‘halves’.
This sort of writing does not, of course, come from conscious counting of syllables, but is organic, born of hours (and years) of trying out and listening to the sound, and getting a rhythm by ear as it were. And of course it comes from listening to a lot of other poets. But it’s interesting to see how such free verses is yet so tight in its impact, and ‘naturally’ falls into patterns.
We’ve looked mainly at the line as a rhythmical unit, and hence at kinds of parallelism. But there are also more random features typical of poetry, where a sound can be repeated just once, and then move into something else.
This can be seen in the use of alliteration, assonance, consonance. In these examples, perhaps you’ll agree, that bits with the same colour have something in common as to sound patterning. So in the Muldoon passage, ‘Amazon’ picks up on sounds in ‘on a’ and then ‘an Indian’ takes them up again. Similarly ‘Indian boy’ partly echoes ‘tributary’ But this is not done systematically.
On a tributary of the Amazon
an Indian boy
steps out of the forest
and strikes up on a flute.
Where once the waters of your face
Spun to my screws, your dry ghost blows,
The dead turns up its eye
In these examples there is an intertwining of sounds which ‘pick each other up’ as echoes, but not overall structural shape. They are ways of getting from one sound pattern to another, and each transition can be based on a different kind of patterning (rhyme, alliteration, assonance, et al). There’s a parallel of this in ideas and images when Dylan Thomas talks of one image ‘breeding’ another in the process of composition.
The relationships here are in some ways similar to those found in cynhannedd, but in the latter things are in fact very strictly patterned. In free verse we often find features of metrical verse drawn on the ways we see in the extracts above.
 This idea is an ancient one, and I suspect of Biblical origin. An interest ing allusion might be to D H Lawrence’s Women in Love where Ursula says
‘Do you think that creation depends on man! It merely doesn’t There are the trees and the grass and the birds. I much prefer to think of the lark rising up in the morning upon a human-less world. Man is a mistake, he must go. There is the grass, the hares, the adders, and the unseen hosts . . ‘