As you can see on the chart below the “Ending Diagonal” can be found at the very end of the five wave fractal, wave 5. All the five micro waves that compose wave 5, impulsive and corrective alike are composed of three micro waves a-b-c. Elliott had to introduce that exception to his own rule that waves 1,3 and 5 can only be subdivided into five smaller waves. And in this case he acknowledged that impulsive waves can be structured as a-b-c in the way how the Harmonic Elliott Wave theory does! You can read more about that in my article. But first let’s zoom in.

You can see on the chart below why I see ES following that pattern. To complete the pattern ES needs to make one more high deeper into the dark red box.
Now you can and should ask me, why I start talking about a pattern recognized by the classic theory?
Because we are not purists here, we are here to learn how to predict the market’s next move. One important element in trading is to stay in observance mode. To keep watching for something that will be recognized as important signs of other traders. Many patterns work as self fulfillment prophecy. 
This “Ending Diagonal” pattern is recognized by tens of thousands of traders who trade the classic version of the Elliott Wave theory as one of the BEST pattern for completion of the final wave v. This is what normally is written in books about that pattern “… when the preceding move of the trend has gone too far, too fast and has run out of steam…”.
When the ED gets completed in many cases price would reverse strongly and fall hard out of the channel.