The word obvious is used because trout need to poke their head out farther to get their mouth over the larger size fly. Sometimes with smaller duns, it is difficult to tell whether a trout is taking a dun or spinner. A fish showing its head, dorsal, then tail is usually taking a dun, but not always. They do also take spinners with a head, dorsal, and tail rise, but it is very often more subtle. Paying attention to the water and seeing what insects are coming down in the largest numbers will often distinguish what a trout is taking.
Spinners are one of the most productive flies on large rivers with good hatches.
Because spinners are dead and lay flat on the water they present an easy meal for trout, which in most cases, they can not resist. A trout feeding on spinners will take head, dorsal, then tail, but at other times will just dimple taking the vulnerable meal. The rise is incredibly subtle, and the size of the fish beneath the rise is often surprisingly big. Large wild trout love spinners, as they can expend the least amount of energy. A trout taking spinners will feed similar to a fish taking duns.