It is this (in some sense conscious) clash between different modes of feeling which is the normal source of pleasure in pastoral; or, at any rate, in so far as pastorals fail to produce it, one may agree with Johnson and call them a bore.This formal relation between 'simple' and 'complex' ('putting the complex in the simple', the closest thing in Some Types of Pastoral to a nutshell-definition of the form, is actually a slogan for all art, as Empson himself knows) stitches form to social relations. Pastoral embodies and therefore enables, as well as representing, a coming-together of poor-simples and rich-sophisticateds.
Thou shalt eat crudded creamThe delicacy of versification here (alliteration, balance of rhythm, and so forth) suggests both the scholar's trained apprehension and the courtier's experience of luxury; but it is of the brambleberry that he is an epicure; the subject forces into contact with these the direct gusto of a "swain." That all these good qualities should be brought together is a normal part of a good poem; indeed, it is a main part of the value of a poem, because they are so hard to bring together in life. But such a case as this is peculiar, because one is made to think of the different people separately; one cannot pretend to oneself that the author is the rustic he is impersonating; there is an element of wit in the first conception of the style. (Seven Types, 114-15)
All the year lasting,
And drink the crystal stream
Pleasant in tasting;
Whig and whey whilst thou lust
Pie-lids and pastry-crust,
Pears, plums, and cherries. (ANON., Oxford Book.)
The essential trick of the old pastoral, which was felt to imply a beautiful relation between rich and poor, was to make simple people express strong feelings (felt as the most universal subject, something fundamentally true about everybody) in learned and fashionable language .... The effect was in some degree to combine in the reader or author the merits of the two sorts; he was made to mirror in himself more completely the effective elements of the society he lived in. This was not a process that you could explain in the course of writing pastoral; it was already shown by the clash between style and theme, and to make the clash work in the right way (not become funny) the writer must keep up a firm pretence that he was unconscious of it. (Some Versions, pp. 11-12)According to George Watson [The Literary Critics (Hogarth 1986), 184] 'Empson has later insisted that his Marxism in the thirties and after -- at least until the Communist revolution in China in 1949, which he witnessed -- was more serious than his writings reveal, and Some Versions assumed the class analysis of society and the ideal status of the "proletariat".' This is true, although the point for Empson, at least in this book, is always to bring the potential for social harmony back into the orientations of individual subjectivity. René Wellek [A History of Modern Criticism 1750-1950: V English Criticism 1900-1950 (Yale 1986), 280] puts forward a slightly more reductive reading.
[The book's] subject is the collapse of the pastoral relation between the swain-hero and the sheep people. It is again the theme of the loss of community, of the presumed original unity which underlies Eliot's concept of history. Pastoral is used in a very wide sense: thus the first chapter discusses proletarian literature which Empson considers a covert pastoral. But even proletarian literature is used in a much wider sense than the usual one ... Proletarian art is pastoral. The old pastoral implied "a beautiful relation between rich and poor"  but this relation has broken down, and the old pastoral had been replaced by the mock pastoral, the comic variety at first. Both versions, straight and comic, are based on a double attitude of the artist to the worker ("I am in one way better, in another not so good"), and this may well recognize a permanent truth about the aesthetic situation. "To produce pure proletarian art the artist must be one with the worker; this is impossible, not for political reasons, but because the artist never is at one with any public." Wellek might have added, though he doesn't, that this is a peculiarly Romantic version of 'the artist', which itself problematises the case being made (something of which Empson himself was aware: 'Mob thought may kill us all before our time, but the scientist's view of it should not be warped by horror, and the writer who isolates himself from all feeling for his audience acquires the faults of romanticism without its virtues.'). Actually, the implied individualism of all this is central to what Empson is arguing: not that the poet is alienated from society, but precisely that the (simple) poet holds within him/herself the (complex) of society. Here's Paul Alpers:
Marvell and Milton represent for Empson a withdrawal-to quote the verses that prompt the essay on "The Garden"-of the mind into its own happiness. The strengths of the "old pastoral" are most fully manifest in Elizabethan works, particularly the dramas, which are discussed in the chapter on "Double Plots." The Elizabethan double plot is a version of pastoral, because it is a convention-the strongest and most capacious, it would seem, in all our literature-for the stable presentation of conflicts and con- tradictions and for putting the complexities of life into the "simple" effects of art. 
"In pastoral you take a limited life and pretend it is the full and normal one, and a suggestion that one must do this with all life, because the normal is itself limited, is easily put into the trick though not necessary to its power" (Some Versions of Pastoral, 114).