many years ago when I was braver/dumber I rode on shoulder of US 40, a 4 lane hi-way. The air coming off any passing vehicle is much like the wake of a boat & when a semi-truck at 60 mph went by it was a big wake. The initial wave of air coming off the front of the truck pushes you forward & then as the truck goes by you're in still draft air. Pedal harder while this is going on & you can upshift 1 or 2 gears & pick up an extra 10 mph for a while. The closer you get to the side of the passing vehicle the greater the effect, & the greater chance of getting plastered which is why I don't do it anymore.