You usually do it when the opponent has or will have enough meter to breaker the combo, and you can hit-confirm and directly follow up b+2,1 and b+3,1+2 with it, or just use it when a normal and hit-confirmed Trance would be used in a combo: b+3,1+2 > b+2,1 > b+1,1 xx EX Trance; (b+2,1)x2 > b+1,1 xx EX Trance; f+1,2 xx EX Trance; 1,1,2 xx EX Trance. It can also follow u+3, f+1, b+2, 2, 1, 1,1, b+3, and b+3,3 or just be done raw, but those are never really used.