'retrospective knowledge', refers to the recollected mental image obtained in concentration, or to any inner experience just passed, as for instance, any absorption (jhāna q.v.), or any supermundane path, or fruition of the path, etc. (s. ariya-puggala).
As it is said: "At the end of fruitional consciousness, consciousness sinks into the subconscious stream of existence (bhavanga-sota, q.v.).
Then, breaking off the stream of existence, mental advertence (manodvārāvajjana) arises at the mind-door, for the purpose of retrospecting the (just passed) path-moment.
Now, as soon as this stage has passed, 7 moments of impulsive consciousness (javana-citta), one after the other, flash up while retrospecting the path.
After they again have sunk into the subconscious stream, there arise, for the purpose of retrospecting the fruition of the path the moments of advertence and impulsion, during whose arising the monk is retrospecting the path, retrospecting the fruition, retrospecting the abandoned defilements, retrospecting the still remaining defilements, retrospecting Nibbāna as object .... 'This blessing have I attained' .... 'This and that defilement still remains in me' .... 'This object have I beheld in my mind', etc." (Vis.M. XXII).