Here’s what comes to mind when I try to come up with a direct way to explain “top-down” writing. Pick and choose words that fit as you see it:
The TOP (melody, gesture, idea, phrase) influences (changes, guides, moves, anticipates, justifies) the BOTTOM (chords, accompaniment, beat, progression, phrase length, groove, feel, rhythm section backing).
When you’re improvising on a jazz tune, and the chord progression is set and unyielding, your solo is moving against the expectations of the progression, your own expectations and reactions, the listeners’, and those of your fellow cats in the band. You can’t “move” the chords, but over time, the chords feed back to you their own tension and resolution, allowing you the opportunity to exploit and enter into a dialogue with this sonic loop; hopefully the other players in the band respond to your ideas and move with you. Or maybe they don’t move…maybe that rigidity intensifies your solo.
We carry the improvising spirit and the freedom of the jazz language with us at all times, and the best writers I know can also play their asses off.
When you’re composing a tune within a rigid song form structure, or for example, arranging a standard tune, you might find clear ways to work within the form to impose your creativity. So you can’t move the chords too much…write from the “top” and guide them. If you have some linear chops you can use good melody, voice leading and counterpoint to imply substitute harmony. If you need the space, add a beat, or a bar or two (or three) to the form…how far you go depends on the commercial and creative confines of your mental workspace, and the demands of the job at hand.
When you’re starting from scratch, staring at the blank page, you have the opportunity to manipulate both strata of expression. I find that just being aware of forward-thinking backward-writing is enough to set me on the right track.