I don't know what you mean by "which other speed reading technique should I use for the Rapid Reading phase."
Rapid Reading really is the ideal way to go through materials when you want more in-depth information/knowledge after activating a book.
I've also found that I don't need to Rapid Read most books, but I do it now and then for pure enjoyment.
And, yes, the more you Rapid Read the better you get with it.
