What comes from taking a dump very hard, often after not shitting for a while; It usually gets unloaded within 1-2 seconds and travels at lethal velocities, hence the name.
On the road trip last thursday, Bob didn't crap for days. But when we finally got to a truck stop, I could've sworn the earth exploded. The stench emanating from the bathroom that housed Bob's velociturds was unbearable. I could even smell that unholy odor for a few hours afterwards.