Freely printed from: https://learn.nimja.com/techniques/fractionation
Fractionation is, at its core, putting the subject in and out of trance repeatedly.
Effectively it's many trances, over and over again.
After trance, most people are very susceptible to going back into trance. Repeating this, even without suggesting it, generally has the effect of making someone 'better' at going deeper.
The repeated waking and dropping into trance also works beautifully as a strong contrast between the two. Allowing the unconscious to mark the trance more easily and go deeper into it.