As I understand it, if you've got a departure out of JFK later than 1900, you'll only be scheduled up to your Table B maximum FDP minus 90 minutes. So in that case you'd basically lose 90 minutes of FDP time in which you could be flying in order to protect against you timing out due to delays (although it's really 60 minutes because there's already a general buffer of 30 minutes).

But I could be reading it wrong, because that memo could really have used some clarity editing.