When she is in Trance having summoned an Eidolon there is a random chance the same Eidolon will appear when Garnet has her next go, even if you make her Attack. It will cost no MP and does not require you to select Eidolon again. The Eidolon will stop appearing when Garnet's Trance bar empties. It is quite useful in long battles, especially if you summon something powerful such as Bahamut or Ark.