For breaking nodes its not the placing OVER THE TOP that breaks them. The fact that most objects ie. metal bars and sheets have more compressive strength than tensional strength means that if you were to push and pull two different anchored-at-one-end bars with the same force then the one being pulled would break force. This is shown in the first attachment.
To break nodes, you need to have ideally a no based anchored triangle (>), with there being an metal bar or 2 down the middle towards the smaller end (->, except a lot closer). Then put the middle bar as close as you can towards the pointy end of the triangle without the end node of the solitary metal bar going over the other side of the node on the 2 connected bars. With a minutes tweaking, most nodes can be broken. Described above is the simplest method, but in the second attachment I show a number of different ways of breaking nodes.
Essentially its putting part of an object so close to a node than when the simulation starts the force acting to stretch the metal is too much and it breaks.
In my attachments, use the frame by frame button (which is "o") from the beginning with the forces view on to see how this works.
Hope my garbled explanation helps Drago!!