Diagrams that exhibit specific interesting features. Here, the entire diagram doesn't have to be top-notch... merely that one feature of it is something that's useful to use in top-notch diagrams.

2D diagrams

• "pull arrows"
  In a few cases, it's important to be really clear about which direction the rope is pulled, to get it to set properly.

3D diagrams

3D animated diagrams

• the line "extends" over time
  This indicates both 1) which end is the standing end and which is the working end, and 2) what part of the line is "finished" in the tying process. It also happens to be fairly easy to implement, allowing the animator to quickly go from a static 3D model, to an animated version.