The Caret Splitter sits opposite the (top of the) Triple Splitter. It is the small section of track which spans the end of the medium marble resting point which does not involve the Triple Splitter. I named it Caret Splitter because it is shaped like vaguely like a caret ^. It is actually shaped more like the Japanese character へ, when viewed from the direction the marbles will reach it.
The plan is to use the Caret Splitter Rudder to push medium medium marbles toward the left (and onto the the Lowest Small-Medium Splitter) and push small marbles toward the right or left, as they like.
Because the medium marbles cannot go to the right, they must set the Caret Splitter Rudder to only send them to the left when they arrive at the Caret Splitter.