Quick answer: Defeat Carmilla and exit her hall on the left. In the adjoining corridor, Rose reaches a ledge with no whip anchor, and Carmilla gives her Aerial Rush. On PC, jump, then press Shift with W/S/A/D while airborne.
Aerial Rush is Carmilla’s power. It lets Rose dash sideways in mid-air, across gaps that are too wide to jump and offer no whip anchor.
Where you get Aerial Rush
Unlocking the ability requires you to defeat Carmilla first. Her hall is a blood-filled room beneath a red moon atop the Library’s Observation Tower. If you haven’t reached it, see the route to Carmilla.
You don’t need a boss, chest or switch for this part. Reaching the ledge is enough to start the scene.
What the ability card says
The card is titled AERIAL RUSH. On PC, it reads:
“Press [Shift key icon] to PROPEL YOURSELF FORWARDS when Rose is in the air.”
After you close the card, a hint stays at the bottom of the screen while you’re in that hallway: “PRESS W/S/A/D+[Shift key icon] TO PROPEL YOURSELF SIDEWAYS WHEN ROSE IS IN THE AIR.”
How to use Aerial Rush
The dash works from a jump, so Rose has to be off the ground before you press anything.
Aerial Rush lets you cross gaps that a regular jump can’t span and where there’s no point to hook the whip. It works alongside the other movement abilities; for the next route, see what to do after Carmilla.
