The bumps are something Donnie takes into account as he keeps low, hanging on to the edge of the window while Serph maneuvers the truck beside his Tank. At least it puts him a little closer at level to the window than if he were on the ground. He just has to make sure not to fall off a moving vehicle again.

"Steady..." He looks ahead to make sure there isn't anything that looks like it might cause some problems before slowly letting go of the window to stand. At least it seems like he's had experience in keeping balance atop a vehicle in motion. He waits until they pull ahead some to take into account basic physics, gauging the distance to the window.

And then he makes the jump.