Tails-out simply means that you store a recorded tape so the tail end of the tape is on the outside of the pancake rather than at the hub. There are a few reasons for the practice.
The first has to do with print-through. All tape has some print-though where signal from one part of the tape creates a shadow signal on the adjacent layer of tape. If you store the tape heads-out the print-through will cause a pre-echo heard right before the song starts. But if you store tapes tails-out the print-through will present a post echo and thus is masked by the audio.
Another reason for storing tails-out is to create a nice even tape pack for storage. After a recording session you should use Play or Spooling mode (if the deck has it) to continue spooling to the take-up reel. Rewind is generally sloppy on most decks, which can lead to uneven tape pack and edge damage in storage.
Finally, itís considered best practice to Fast Forward or Rewind a new tape before a recording session. If you store your tapes tails out all you have to do is mount it on the take-up side and rewind to the supply reel. I mount all my new tapes this way as wellÖ put it on the take-up side and rewind before recording.
"If you canít make a hit record with a Tascam or a Fostex,
then youíre not going to able to do it with a Studer or Otari!" -David Mellor