"Contrast with part 2

Airblast deflection makes any projectiles inside the 256 x 256 x 128 HU box in front of the pyro switch direction so that they move toward the point in the world where the pyro's crosshair is aiming. A trace is emitted from the pyro's eye position in the direction of his aim to detect the spot he's aiming toward.

This diagram shows how, under normal circumstances, you'll be aiming toward a relatively faraway object, and the projectile will be relatively close to you when it changes direction; so it will be reflected in roughly the same direction that you're facing.

The three X's at the bottom of the whiteboard show the relative horizontal positions of the pyro (black), projectile (red), and the trace hit location (green). Notice that the projectile is significantly closer to the pyro than the trace hit.

Watch this video too:
https://www.youtube.com/watch?v=VflewO6lSI8"