if you regard the last of the "neck" positions being Simandl 5th Position (pinkie plays F on the G string) then it is the transition area between that and thumb position (side of thumb plays G octave or higher). The LH is turning progressively more towards the bridge, coming forward and standing up higher to avoid the forearm grounding on the ribs and breaking the flow of movement.
Two other things are happening in this area. The thumb pad leaves the back of the base of the neck and tracks diagonally down as it prepares to flip over onto the strings. The pinkie pulls away from the strings as the hand turns and is replaced by the 3rd. (ring) finger. Depending on your hand shape and the length of the neck you might still be able to use your pinkie for F sharp. The notes are now so close together that the 3rd finger is just as good or better.