Cruise drain is where you activate your cruise engines and your energy goes down to a certain percent and STAYS THERE for the duration of the cruise and for a few seconds thereafter. For fighters up to Transports, it's around 10% and like 10 seconds. For Gunboats on up, it's literally down to enough to fire one Solaris for like 20 seconds. I think a dev with access to the FlHook code can tell you better than I can.
Found both. They were in that dsacesrv.cfg that Alley had me edit today. The code isn't word-wrapped all neat but jumbled up in the file. I put in a search and found the multi-box and the drain on cruise.