Create a new point, then right click on it to open the "Edit Point" dialog. Check the "Fixed"  box, then click "OK". Try to move the point by dragging with the right mouse button. The point will not move.

Duplicate the first (upper) object at left  in the empty area of the construction window. Use only two segments in your construction, like a "T" . Create the top of the "T" first; then place one endpoint  of the second segment on the first segment (make sure the segment highlights before you click to create the point).

Try moving the  point of intersection of the two segments (drag with right mouse button). The point remains on the first segment.

Right click (do not drag) on the point of intersection, to open up the Properties dialog. Click "Release Point," then move the point again. Now the point can jump off  the segment.

Now duplicate the lower object at left. First create two crossing segments. Then creat a third segement with one endpoint at the intersection of the first two.

As you move the endpoints of the two crossing segments, the point of intersection moves, and so does the third segment. If you move an endpoint so that the segments no longer cross, then the third segment disappears entirely: it is no longer valid.

It's possible to move multiple points together using the "Move" tool as follows:

•  First select the "Move" tool.
• Then hold down the "shift" button and click on several points in the window. If the point is moveable, it will be highlighted in red (for instance, that the intersection of the three segments is not moveable).
• Finally, release the "shift" and place the mouse over one of the highlighted points so that the cursor becomes a hand. Left click and drag to move the selected points.

Try again to move multiple points as in the previous paragraph, only this time when you release the "shift" hold down the "ctrl"  button while you drag the points.You will see that the original state of the construction remains visible as you move the points. When you let go of the mouse, the original state disappears, and is replaced by the new construction.

Notes:

•  Typing "m" selects the "Move" tool.
•  When the "Move" tool is first selected, all movable objects are highlighted. The "Move" tool has no effect on objects that are not moveable (for instance, points of intersection between crossing segments).

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