Yang Jun talked about the sounds Heng, Hai, and Ha very briefly in class one night about four years ago. I will relate what I remember and my impressions. At that time I had no understanding of fa jin at all, so my impressions should be treated with much caution. To date, we have not used these sounds in class ever. Most of us are not accomplished enough to fa with sufficient sung, including myself, so I suggest you ask Yang Jun about these sounds if you have the opportunity.
Heng is the sound one makes when someone is striking your torso. It is an inhalation designed to give you a little more space. Here Yang Jun hunched his back as though wrapping his torso around a punch to the belly. It may be a way of sucking air into the upper lungs to avoid being winded. It sounded like someone desperately gulping for air after being too long under water. Or perhaps the large and comically exaggerated breaths taken before diving when clowning around in front of friends. It didn't sound to much like "Heng" except insofar as wheezing in air--ah, I've got it: It sounded like an asthmatic hyperventilating.
I think as far as internal elements go, Heng may have to do with absorbing, or yielding. I think it's a kind of yielding with the breath so that your opponent cannot attack your breath. It's getting your air/breath and thus yourself out of the way so they cannot attack your breathing.
I say inhaling, but I'm not 100% certain that it couldn't be done while exhaling--if someone strikes and you make this noise, it could possible keep you from being entirely winded even if some air is forced from your lungs.
IIRC, this is the sound made with penetrating fa jin. The kind that is directed at the opponents internal organs. We were told not to use this one on each other as it was too dangerous. Quite probably, he meant using penetrating fa jin on each other, and not the sound. Without the action, I'm guessing the sound is useless. But he demonstrated (with no opponent) and even the sound was striking even though I was about 10 feet away.
The closest comparison I've heard is the Karate "kiaii" made when striking--but I have never heard a karate student produce a sound like this. Another similar sound is the Japanese "Yes" (also Hai) as pronounced with vigor and enthusiasm in the movies by Japanese men when answering their bosses. But tai chi Hai has no curtailment, no glottal stop.
This one is also for fa jin, but for the more friendly long jin (?) that pushes people out but doesn't scramble their internal organs. This is the one where the opponent's hands can go "whap" against your chest and you get tossed out or up in the air but there's no damage. I think this jin passes through but does not linger.
It sound's like an exclamation of victory, "Ha!" Imagine someone who has one a game yelling "Ha! I win!" It sounds something like this. It can be breathier when pushing slowly or explosive when using fa jin.
[This message has been edited by Kalamondin (edited 09-26-2005).]