Increases Handling while boosting.
Mk1: Handling +195
Mk2: Handling +218
Mk3: Handling +263
Mk4: Handling +323
The "Velocity: Handling" trait can be very helpful to run away, allowing to perform risky high-wire acts with terrain features more easily.
It does require some practice to pull off, so I strongly suggest doing so in single player practice matches and just continually boost through terrain with this trait.
After some practice, you'll be surprised of what you will pull off in real games afterwards ;)