The word 下降 is formed by combining two characters, each contributing its core meaning to create a new, unified concept. The first character, 下, means "down" or "below", establishing the direction. The second character, 降, means "to descend" or "to lower", providing the specific action. Their combination is not grammatical but purely semantic, merging these two related ideas into a single, more vivid and complete notion of "to go down" or "to decline", where the whole meaning is the direct sum of its parts.