Your Thoughts On "Heng", Crossing Fist?

Xingyi Five-Element Forms (slightly different than the way we practice it)

Regular readers probably know that in an effort to expand my understanding of Chinese internal arts, I have been learning the Xingyi 5-element forms. In order to stay within the framework of internal arts, I've had to correct previous habits from Karate, such as avoiding full chambered punches.
The first four elements- splitting, smashing/crushing, drilling and pounding are pretty self-evident in performance and application.
But the Fifth Element (Hmmm, a Bruce Willis movie) is a little confusing and I'm interested in other people's opinions. It is serpentine in nature, and the application is often shown as a strike that curves around an opponent's arm with striking surface being the thumb side of the fist. I'm not sold on that yet.
In reviewing my reference material from a recent Seminar that Tim Cartmell gave on the similarities between Taiji "Peng", Bagua "Fan" and Xingyi "Heng" (Crossing Fist), Tim demonstrated that all three related techniques use alternating arm circles in (Peng) rising, (Fan) oblique, and (Heng) horizontal. Same basic concept in each.

Above is an example of the type of use for Crossing Fist that Tim Cartmell teaches, presented in my own clumsy interpetation.
As my Xingyi instructor Jake Burroughs explained, a significant feature in this technique is not the striking out with the fist. The emphisis is rather on the hip action and setting into the Kua. Jake warned me that learning crossing fist correctly was going to drive me crazy, so I welcome comments from people out there familiar with the form. One more thing:
The Crossing Fist, Heng, is said in the classics to be the "mother" of the five elements. Tim explains that this is due to the nature of the alternating arm circles noted above. The Classics also mention Heng as related to Earth and the Spleen, which reglates Yin and Yang and therefore regulates yin and Yang of the other four fists.
If other people more experianced than myself would like to give comments/opinions on the subject of Heng, Crossing Fist, It would be greatly appreciated.

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Thanks for all your suggestions and opinions on Heng-
If you haven't read it yet, Jose' provided one of the best descriptions of this technique that I have read yet. Jose' - as always, thank you very much!