<p>I wouldn't classify suffocation as a subskill, per se. It's still normal airbending, just used in a violent way that we haven't seen before. Subskills tend to involve aspects of an element that are fairly different from the original (Fire and lightning, earth and metal, water and blood) or have more magical effects using the parent element as a medium (healing or spirit energy conversion in waterbending). The air Zaheer bent out of the Earth Queen's lungs wasn't any different than normal air, and there were no additional effects present in his bending.
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