Does "which is void of formations (conditioned phenomena) since it is the cessation of them" refer to nibbana or the knowledge that abides in nibbana? The essay The Progress of Insight includes the following definitions:


  Maturity Knowledge
  
  Immediately afterwards, a type of knowledge manifests itself that, as it were, falls for the first time into Nibbana, which is void of formations (conditioned phenomena) since it is the cessation of them. This knowledge is called "maturity knowledge."
  
  Path Knowledge
  
  It is followed immediately by knowledge that abides in that same Nibbana, which is void of formations since it is the cessation of them. This is called "path knowledge."[43] It is also called "purification by knowledge and vision."


In the first paragraph, does “which is void of formations (conditioned phenomena) since it is the cessation of them” refer to nibbana, or does it refer to the knowledge that abides in nibbana?
The roots (greed, hatred, delusion, non greed, non hatred, non delusion) creates conditioned fabricated state. When the root condition passes away so does what was fabricated. The root conditions are impermanent hence is the result. Each root condition and the result is tied with sensation which are unsatisfactory they neither last or in control of them.

When you do Vipassana and keep eliminating to roots a stage will come where there is no more results of past conditioning nor is you mind creating new conditioning. At this state you experience the 1st glimpse of Nirvana. As the root and result is tied to sensation what you experience as Nirvana does not have any sensation hence is satisfactory (not unsatisfactory) and since not dependent on an impermanent root it is permanent though still it is not not in ones control (you cannot bestore it on someone else nor can you realise it by will or wish).