Creating a vacuum isn't that hard to conceptualize. It's moving air out of a location without allowing any back in. Really can't use it for very long.

Sadly, the air scooter could be the specialization.

I disagree with that. All the other mentioned specialties are uncommon and/or dificult to attain. however, as we seen in the episode where we learn of Aang's past, we see as all the students learn the "trick" in almost no time at all.

OK, I read all the posts. I agree that there is nothing special about bloodbending. Its just bending water that is not usually bended. Healing is obviously waterbending's special ability. It is the only thing different that a waterbener can do. Everything else is just water. Lightning is ofcourse firebending's special ability. And for those who said "lightning bending", I thought that lightning isn't really bended but just created and discharged. Thats all that is done, there is no further manipulation of the lightning.

As for airbending, well there either isn't one or it has not been shown. Personally I think its too late to see it becuase prabably only a select number of master airbenders knew it, and they were wiped out before Aang could be tuaght it anyways. We can only fantasize what it could be. I think sound is a good candidate. But I think its more of a sonic boom or sound wave thing that can crumble walls rather than actual "sound bending". Something I thought of was compressing air into a solid or liquid, perhaps freezing it and creating something similiar to nitrogen. And with that I mean a really cold liquid. And the airbender can still bend liquid oxygen becuase it is composed of what "air" is made of, despite the fact that its a liquid now. Its like waterbenders bending ice even though its a solid. But all that sounds a bit farfetched, so lets stick with the sound thing.

As for earthbending, it prabably has not been shown. People say metalbending but that is just like bloodbending. Metal bending is just bending earth in a place no one usually tries. And it is only something Toph can do since she is soo powerfull. (The blindness helps too) Others have said creating earthquakes, but once again thats just regular bending but on a large scale. Perhaps the special ability has to do with tapping in to the earth and using its abilities aswell, such as gravity, or magnetic feilds. As for what gives earthbenders their powerboost, Ill just guess its either the Earth itself or massive tectonic shifts. Maybe all that earth moving gives earthbenders power.

And as for the airbenders' power boost, I think that has to do more with the weather. And I dont mean a hurricane or tornado, I mean more technical stuff like air pressures/temperature and weather fronts and stuff.

I've something, we already know that both metal and bloodbending are based on an element that they can control small particles in something bigger, well, I think that the airbending sub-element is reverse, well, by example, the airbender can make a transposition into another body, making the other moving and acting like he wants. (oupss, did I go too far ? )

lol back to the topic....

I think this whole crazy "sub-bending" thing is totally ridiculous, it makes the show sound like pokemon

gas bending, using the chemicals in the air to make poisonous gas

I still think creating illusions is another possibility besides sound.

and for earth it has to be something rigid, maybe stone skin?

ok from reading all of these comments i think we all agree that waters "sub-bending" is healing no doubt. Metal dose not have one yet. yes metal could be but it still has the basic use of earth. Fire is also no doubt lightning. Last but not least air dose not have one. The sound of vacums sound really cool. every since i was little i have always thought that if you tooked the air out a certain place and put something in it. it would be like space but now thinking about it gravity would still be affecting were it is. sound waves would be cool but it wouldent be "sub-bending" because the molecules, yes, still affect sound so really. In the end there is no air bending "SB" it would be cool to see it.

well, obviously sub-bending is how benders make a hoggie.