The seam
Wherever current speeds differ, a visible line forms. Fast water delivers food. Slow water costs nothing to sit in. A predator holds in the slow side of the seam and eats out of the fast side.
Where seams form
Behind boulders. Downstream of bridge pilings. On the inside of every bend. Where a tributary enters. At the tail of a riffle.
Presentation
Cast upstream of the seam and let the current carry the bait into it. A bait that crosses the seam naturally gets eaten. A bait dragged across it does not.