Slice Tool

static/cursorSlice.png

Will add points on dragged slice line.

static/sliceLine.png
click add a starting point of a slice line
drag sets end point of slice line
drag shift adjust slice line on x, y or 45°
mouse up slice the glyph according the slice line

When the slice line has 2 intersection on the same contour it will cut the contour, otherwise is will just add points. (Guides can also slice see Guides)