To break open a run at the cursor, press the letter "O" key. You can control at which end of the module the break happens by placing the cursor nearest that end.

Even though there is a menu item for Break Open, this is a function that you want to use with the shortcut key because the position of the mouse is important.

In the run on the left, the mouse position was below the break point when "O" was pressed, and in the run on the right, the mouse position was above the break point.