I don't get when or how to use this Technique from Path of Waves.
The attack roll is against Vigilance, and you get the disoriented condition. And it makes Fatigue damage+1 for each extra two successes, with the possibly of spending opportunities for doing a crit if you incapacitate the opponent.

But we already had the Crossing blade, that was against TN2 (usually lower than Vigilance) and did damage equal to deadliness + 1 for each extra success (it doesn't crit if incapacites, but takes the opponent out of combat anyway and you can kill him easily next round if you want to). Or the Rising blade, that crits automatically if the opponent is incapacited. All of this at rank 2 instead of 4,and with no need to become disoriented after the attack.

What am I missing with Reverse draw?

If you're Compromised (which would trigger Final Blow in a Duel), your Vigilance becomes 1, so Reverse Draw drops to TN 1. But, Rising Blade doesn't scale well, unlike Crossing Blade, so Reverse Draw kind of combines the two into a difficult technique you don't want to use unless you'll be taking care of your opponent (either in a Duel or Skirmish), but combines positive aspects of the previous two Iai techniques to have the ability to lower TN and scale damage with success. Narratively it also presumably looks more impressive if you pull it off.

5 hours ago, Sedda said:

Or the Rising blade, that crits automatically if the opponent is incapacited. All of this at rank 2 instead of 4,and with no need to become disoriented after the attack.

What am I missing with Reverse draw?

All attacks crit automatically against an incapacitated opponent. I think you meant compromised there.

And as UnitOmega mentioned, it takes the best of two different techniques and puts into one.

This. Rising blade is great against a compromised target as a finishing blow, Crossing Cut is good for incapacitating an opponent by smacking down lots of fatigue.

Reverse Draw is great for a target that's nearly incapacitated, letting you draw a blade and potentially critical strike the target in one go.

15 hours ago, Sedda said:

(it doesn't crit if incapacites, but takes the opponent out of combat anyway and you can kill him easily next round if you want to)

Remember in a 'formal' duel you lose points for every strike needed after the first.

We've had some extended discussions on the Discord about it, too. We've currently found the following niches for it:

If you are a bushi or shinobi who isn't focusing on iaijutsu dueling, you can learn one tech which can sorta take the place of two other techs. It's worse at it than either one, but it can fill both niches.
It also can threaten high damage and a high severity crit, potentially forcing your opponent to spend a Void Point to just take a weaker crit.

It's also okay in cases where your opponent already has some fatigue, like a battlefield clash.