If you are talking about the finishing move for espers. Then you have to know that the ones that you get in the storyline which are Mateus, Belias, Hashmal, Famfrit, Shemhazai, their moves activate when your HP is low and the summoning time is less than 10 seconds. The optional espers have special way of doing their best move, like you need to have like a certain status effect or something like that. And to activate Ultima's move, you and her need to have critical HP