Trapping is a print production technique where a slight overlap is created between colors that are adjacent to each other. Effectively where the colors overlap and bleed onto the other overlapping extra shapes onto the graphic behind. Using the pathfinder to subtract the shape form the shape of your stroke. And here is the settings to do this, Add a stroke to your object, then convert the stroke into a path:
object > path > outline stroke, fill the outline with a mix of the two colors
needing a trapping.
Trapping/Partial Overlap.
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