To attach a point to a particular object, first click on a free point and then on the object to which you want to attach this point. The point becomes dependent and can only be moved within the object.
Dependent points can be detached with detach. After selecting this tool, just click on the appropriate point. The point becomes a free object and can be moved anywhere within the drawing area.
Video presentation of the basic Attach / Detach Point tool usage.
Source files for video presentation.