Harley Quinn is on her skates being towed by rope behind Huntress' bike:
Harley Quinn: Whip me!
Whipping in this case is a maneuver whereby the speed and hence momentum of a skater is increased by pulling them forward past an the already fast moving person performing the whip. In Harley's position she would be whipped by Huntress, causing her to pass Huntress and be moving faster.
There's a basic description under Strategy and Tactics\Jamming on the Wikipedia page for Roller Derby.
