The start of Robert Frost's 'Directive' (which Jarrell has a long appreciative essay on - hence my doubts) goes like this:
Back out of all this now too much for us
(the whole poem is here: http://hjem.get2net.dk/abra-ken/Frost1.htm)
What I'm interested in here is metre. Its underlying pulse, confirmed by the rest f the poem, is iambic. I suppose you'd scan it, crudely, something like this:
Back OUT (or BACK out) of ALL this NOW too MUCH for US
Even on a first go, the first foot is causing problems. Neither Back OUT nor BACK out is quite right; in fact we want to read it as a spondee: BACK OUT of ALL this... But the same considerations apply throughout the line. In fact, there's a case for reading it as follows, with a whopping eight stressed syllables:
BACK OUT of ALL THIS NOW TOO MUCH for US
I'm no metrical expert, and I shan't introduce here notions like relative or graded stress. All I want to notice is the sheer heaviness, the stuffedness, of this line, which offers us only two syllables (of and for) which could be called slight or modest or recessive. The rhythmical density combines with the syntactic density (what does the line mean? or, since the meaning is fairly clear in context if not paraphrasable, how should it be parsed?) to give a puzzling, mesmerising, dreamlike effect. It sets the tone of the poem and also introduces us into its technical style, a kind of seat-of-your-pants pentameter. Contrast it with the first line of Tennyson's 'Tithonus', from the opposite extreme:
The woods decay, the woods decay and fall
(the whole poem is here: http://charon.sfsu.edu/TENNYSON/TITHONUS.HTML)
Here the metre is strictly adhered to, and similarly, announces the poem which follows, a classical and classically beautiful piece of verse. It has none of the heaviness of Frost's line, and none of the syntactic forcing - indeed, the repetition makes the syntax still more lucid and Latinate, in contrast to 'Directive's Anglo-Saxon modernity.
Note that 'the woods decay' takes a subtly different meaning in its two occurences. The first time, it means what it says literally, and also expresses the idea of degeneration. The second time, it is part of a conjunction ('the woods decay and fall') which itself expresses the same idea; but the literal meaning ('the woods decay') is only a part of that conjunction: the thought is complicated. Specifically there is the suggestion of time passing (a major theme of the poem): at first the woods only decay, then they fall.
It's true that such an analysis is rather precious, but I think it's worth making because it spells out what I mean by talking of the Latinate syntax, which is lucid without being banal (the repetition isn't mere repetition). And that recognition of syntax as being a factor in the way metre is fulfilled brings me on towards thinking about lighter lines. If Frost's is heavy, with eight syllables bearing stress, and Tennyson's is standard, with five, what sort of line could bear fewer than five stresses and still fit in a passage of iambic pentameter?
I'll assume that a line of iambic pentameter that had only two or three stressed syllables would, in most cases, simply be a weak line of poetry, even if it could be constructed. But four seems more promising. Specifically (and this goes back to what I was saying about syntax above), I think Augustan satire makes use of the four-stress line, probably because the style requires the sense to be more or less contained in the line, or at least in the couplet, so syntactical constructions which allow this are brought in at the expense of the fifth stressed syllable. It doesn't matter, because the metre is so strongly established that the missed beat is accepted and even adds value through syncopation. Mainly I think it happens on the medial stressed syllable, where words like and, but and of are used to conjoin two ideas - here are some example, all drawn from the first canto of The Rape of the Lock:
The silver Token, and the Circled Green
The light Militia of the lower sky
When Music softens, and when Dancing fires
In each case, the conjoining syllable, though it does bear stress, recedes in the face of the much stronger sounds on either side.