Number of lines which must be kept together at the start or end of a paragraph. If there are insufficient free lines in the current frame to start the paragraph, NewLeaf will skip to the next frame. If the split would leave fewer lines than this value at the top of the new frame, NewLeaf moves the breakpoint forward until enough lines come beyond it.

If it is impossible to satisfy both conditions (there may not be enough lines in the paragraph) the split will occur approximately 2/3 of the way down, i.e. the majority of the paragraph will be placed at the foot of the previous page.

leaf.Keep 6  © At least 6 lines

The default setting is to skip if fewer than 2 lines are free.