Dawnwing has a gauge near the middle of the screen that fills.
This gauge fills slowly when in combat, or quickly when shooting.
(Max is 100. Gain 10/s naturally, as well as 4 per shot hit.)
When full, the next shot Dawnwing fires will reset the gauge and trigger her Crystal Vanes - Up to 6 Crystal Vanes will fire at the shot enemy, dealing high Frost damage.
After proc'ing the passive, some Crystal Vanes disappear (how many disappear depend on how many you have. With 6 or more Vanes up, 3 Vanes will disappear)
The damage from her Vanes proc'ing and firing at enemies is where most of Dawnwing's damage comes from.
Since a maximum of 6 vanes proc, having more than 6 vanes up doesn't increase the damage of Dawnwing proc.
You can use periods of downtime (e.g. enemy invulnerability phases) to build vanes, so that you don't need to make them during combat and can focus on just on proc'ing the crystals rather than making them, which helps Dawnwing's dps a little.
For each Crystal Vane that fires at an enemy, Dawnwing recovers 4 bullets.
With the max of 6 Crystal Vanes firing, this means Dawnwing recovers 4*6=24 bullets per proc.
Due to Dawnwings natural aurora regeneration (10/s) means it actually takes her a little less than 24 bullets to proc her vanes, which recover 24 bullets.
This means that [provided you land your shots] Dawnwing never needs to manually reload.
Shooting an enemy builds the gauge, even if the enemy is invulnerable.
This allows you to build the gauge to full whilst an enemy is invulnerable so that when they become vulnerable again your next shot proc's Dawnwing's Crystal Vanes.
Note that in practice you don't need to worry about her Crystal Vanes - just use her standard skill to dash twice after the vanes fire, and you'll always stay at or above 6.
Dawnwing will also use her standard skill automatically when switched in (This doesn't use any S-Energy or skill charges though)
This does create Crystal Vanes, but since it's her standard skill it does also cause her to move, which can make it a liability in certain situations.
Especially if you only move once and don't dash twice to end up back where you started.
Note that for Dawnwing to double-dash when switched in you need to hold her standard skill key (Default: E) down, not her swap-in key.
This does require some practice.