Continuity mistake: When Scooby-Doo is facing one of the monsters he bites the fingernails off its left hand, a moment later it lashes at him and the finger nails have grown back in perfect condition again.
Continuity mistake: After the scene where the gang has each other's voices, they are seen walking to the Voodoo man's house because they hear an explosion. Fred is holding Daphne's purple purse in the forest, then he isn't carrying it at the Voodoo man's house. Then in the next scene, Daphne has it back again.
Continuity mistake: In the scene where the van crashes into the dock's factory doors, the van is stood there in the factory. But when it cuts to outside of the factory, the van is on the road, and no-one drove it there.
Answer: It's never explained. We're just left to assume he was somehow able to.