In Federal Vision theology, what is the difference between decisional regeneration and presupposed regeneration?

In some Reformed/Calvinist circles, a theological framework called the [Federal Vision](https://en.wikipedia.org/wiki/Federal_Vision) (FV) has led to significant internal debate over the true nature of the covenant between God and man, and, by extension, the role of faith and works in justification and salvation more generally. I'm currently reading a book by an FV opponent, David J. Engelsma, called [*Federal Vision: Heresy at the Root*](https://books.google.com/books?id=SqTGMQEACAAJ) . In chapter 6 he seems ready to address the following challenge: > Some of the proponents of the federal vision are decisional regenerationists; others hold to presupposed regeneration. How can you say that both hold to the same view of the covenant? But Engelsma's response does not shed much light on the difference between these views – he simply continues to group them together and critiques FV more generally. That's less than satisfying, so my question here is: **according to FV proponents, what are the perceived differences between decisional regeneration and presupposed regeneration views?** What impact do these differences have on the doctrine of the covenant held by different FV proponents?