"Slingshot Engaged"
"The low-pressure wake behind a group's leading car reduces the aerodynamic resistance on the front of the trailing car, allowing the second car to pull closer. As the second car nears the first, it pushes high-pressure air forward so less fast-moving air hits the lead car's spoiler. The result is less drag for both cars, allowing faster speeds.
"Usually the cars following aren't wide open throttle like the lead car of the draft is, UNLESS they are bump drafting. In "normal" drafting, both cars are going the same speed, but both are using different amounts of throttle. Unfortunately the front car is giving it all that it can, while the others are using about 60-80% throttle (that's the benefit of drafting). That being said, the following car can floor it, break free from the draft and attempt to make that pass."
https://answers.yahoo.com/question/index?qid=20100606123832AAdb18m