In a long-filler cigar the roller folds whole leaves so their channels run head to foot, which is what allows air to travel evenly and the coal to advance in a flat line. The skill involved is the reason "totalmente a mano" — totally by hand — appears on boxes.
Short filler and the Cuban sandwich
Short filler is chopped scrap, often the offcuts from long-filler production. It packs unevenly, burns faster and hotter, and is what most machine-made cigars contain. A "Cuban sandwich" is the middle case: short filler held in a long-filler binder, which draws better than pure short filler and costs less than pure long.
Why it shows in the ash
Long filler tends to hold a firmer, longer ash because the leaves themselves are continuous. Short filler sheds. This is a genuine structural difference, though ash length on its own says nothing about quality.