6. The Tools in the Toolbox / 6.4 Tools for Vector Layers
This tool allows to insert additional points in a vector path.
By clicking with the hammer cursor on the path, the new point is inserted. In curves a curve point will be inserted and in lines a line point. Holding Ctrl (Windows) or Command (macOS) reverts this behavior.
Whenever the mouse cursor is on a position where a point can be inserted, the cursor changes to another hammer form.