Good question, hojan.
The fundamental difference between ‘nirodha samāpatti’ and ‘phala samāpatti’ is the following.
In nirodha samāpatti, there is no flow of citta, and the mind is not even in the bhavanga state. One is “not in this world” during that time and that is the closest to experiencing Nibbana.
– Of course, only an Arahant who has cultivated all jhana can get to nirodha samapatti.
– Just before getting to nirodha samāpatti, one is at the “neva sanna na sanna” stage and only a trace of sanna is left there. Then one makes a determination of how long to be in the nirodha samāpatti and enters nirodha samāpatti.
One who is in a phala samapatti has citta flowing with that “phala citta.”
– That is very similar to the uninterrupted flow of “jhana citta” for one in jhana samapatti. That is discussed at the end of the post “Citta Vithi – Processing of Sense Inputs.”
– One who have cultivated jhana can get into jhana samapatti, but NOT nirodha samapatti.
There was a long discussion on the subject sometime back: “Difference between “Arahant phala samapatti” and “Nirodha samapatti” ?”