How do i do showtime mode on burnout paradise the ulitmate box (pc)?

okai so i am trying to do showtime i have ined z as showtime and moved the brake to s so switched then round but i cant do showtime when i get to a junction or what ever i press the on (whilst still moving) and nothing happens why is this and how can i do showtime is there any specific reason why i cant do it i ahve lisence d as i have only just bought the game?