Well, Tom's got me reprocessing this theory before I've even exchanged a pawn for a participle, but let's continue.
In chess, the transition from opening to midgame is marked by an exchange of pieces, intitiated by the sacrifice of a piece for the purpose of clearing out and ultimately assuming a controlling posture toward (though not necessary controlling position in) the middle of the board. It is marked by the beginning of development of more powerful pieces for use in midgame.
Before we get to the development of the powerful pieces, let's discuss the transition, the exchange. How is this akin to writing poems? My contention is that more successful poems tend to operate from a vulnerable position - confessional exposure being the most obvious and most overused. If you're willing to think ahead with me and anticipate that a good poem has to leave itself open in anticipation of a surprise somewhere in it (like a well-played chess game will at some point deviate from mere parroting of the great players), then the first transition of that poem is the conscious direction that creates the opportunity for surprise. The sacrifice of a bishop to radically and blatantly disrupt the center defenses may be similar to the driving home of the poem's idea through a repetition of word and image.
Or, maybe, the transition is the deliberate clearing of the space around the poem's opening that makes taking it in a new direction possible. On the chess board, this could be exchange of pawns and knights that leaves the middle empty for the queen to take over. In the poem, this is discarding of the details not central to the poem's thesis that leave the metaphor available for detail and embellishment.
Just like the meaning of a chess move can only be interpreted in the context of the style of the player, the meaning of a poetry development can only be interpreted in the context of the style of the poet. Similarly, a single poet's/player's style can change from poem/game to poem/game.
Next up: me bouncing off your comments.
Following that: Midpoem.