[MauranKilom]

Close, but not the full truth.

The trigger looks for when you start the animation of the spell. Since Windrunner has a 0.3 cast point, she will start with 0.3 seconds of "charge". She will still channel for 1.0 seconds after reaching her cast point, meaning you have a total of 1.3 seconds of "charging" time, but the damage won't increase after 1 second.

So the timeline goes like this:
0.0s: WR starts the animation of Powershot.
0.3s: She reaches her cast point. Mana is spent and spell goes into cooldown. If canceled exactly now, you'll deal 0.3*maxDamage to the first target.
1.0s: Maximum damage reached. Charging after this won't increase the damage.
1.3s: Powershot is released automatically. Still does max damage.

Aside from that BorisPeace- is right about the linear scaling though. The "channel time" simply starts earlier than intended though (you might call this a bug).