I'm thrilled with what I'm seeing here, but there's one specific bit I haven't been able to find in the rules.

Many techniques offer rolls to resist their effects. However, in the technique listings, these are denoted like 'TN 3 Fitness (TN 4 Earth, TN 2 Air).' Neither the general rule for resisting on page 16 nor the sidebar on page 108 address what the numbers in brackets mean. Is the TN modified based on the stance the defender is in?

Thanks, and apologies for my blindness if it's something I've overlooked!

I'm pretty sure the player says how they plan to resist the effect, which then gives the ring that they'll use. Then they roll it with the relevant skill (if any) against the tn given to them. The TNs in the parentheses are to show that resisting whatever that example was is easier with Air and harder with Earth. Similarly, I could legitimately flex away people who want to beat me up, chances are my muscles will not impress though, so it would be harder than doing a rad flying spin kick at them to make them think I know how to fight.

I actually had the same question. I assume one is the most optimal and one is the least.

And any approach not mentioned is at the difficulty described. Yep; that's my reading too.

It is the standard TN for resisting the effect, then modified TN if the target is in one of those Stances. Stance persists for the entire round, until a characters next action, so there's basic advantages to Stance but in this situation Air has a second benefit with the reduced TN. A target who chose Earth would in this case suffer for choosing that stance.