Update README.md

README.md

@@ -561,7 +561,9 @@ There are different edit modes available, shown on the right side of the screen.
   Regardless of the current select mode, it is always possible to select helices in the side view. Pressing delete will delete those helices. You can also delete a helix by clicking on the helix in the side view while in pencil mode (see below).
 
 * **(r)ope select:**
-  This is similar to select mode, but it allows one to draw a general polygon (a "rope"), rather than just a rectangle. The interpretation of Shift and Ctrl are similar (Shift means add all items inside the polygon to the selected items; Ctrl means toggle them). First, press and hold either Shift or Ctrl. While holding down the Shift or Ctrl key, click several points to define a polygon. 
+  This is similar to select mode, but it allows one to draw a general polygon (a "rope"), rather than just a rectangle. This is useful, for example, for selecting many objects along a diagonal, where a rectangle containing all of them would also contain many objects off the diagonal that are not intended to be selected.
+
+  The interpretation of Shift and Ctrl are similar (Shift means add all items inside the polygon to the selected items; Ctrl means toggle them). First, press and hold either Shift or Ctrl. While holding down the Shift or Ctrl key, click several points to define a polygon. 
 
   ![](images/screenshot-polygon.png)