To "Ghost Ride" is to exit a moving vehicle and dance around and/or on it. You often hear about this in terms of "Ghostriding the whip." Whip is a car, generally a crappy one, like a scraper.

References related to Oakland:


A's Closer Sean Doolittleghost rides: http://www.theplayerstribune.com/sean-doolittle-athletics-9-things-id-like-to-ride-to-the-mound/