The image of the circle highlights a number of issues, the first being that there is no set place to get into the hermeneutical process. The line of the circle has no beginning. We each enter the process as we are able from where we are. Bultmann referred to the existing understanding as preunderstanding to show its provisional nature. It is provisional because it needs to allow the new reading of the text to modify understanding.
Every understanding is provisional, the preunderstanding of the next reading.
No reading is ever final. That is the second point made by the image of the circle. The reader goes round and round but each time around is new. That is because Bultmann recognised that the interpreter today is worlds apart from the texts of the New Testament. At the same time I note Jeffrey's reference to the study of the Greek text, the very reading of which presupposes careful historical linguistic work.