The Fingering Techniques
When two manual parts are notated in the treble staff and another in the bass staff, as in four-part hymns, the right hand usually plays both treble parts. However, the left hand can often help by playing some of the “inner” (alto) notes. This redistribution of the inner part between the hands makes it easier to play legato connections when there are more than two manual parts. It is similar to direct fingering in that it rarely involves crossing, glissando, or substitution, but it is more complex in that the left hand “jumps” in and out of the treble staff.
Drawing a line in the treble staff that "scoops" notes into the left hand (see yellow highlight above) is a common way of notating redistribution.
See the Redistribution lesson.