Well it says " [color=#A99877]transform into an angry chicken for up to 5 seconds that can explode for 215% weapon dmg as physical to all enemies within 12 yards"[/color] So I'm assuming that you get an aoe ability for 5 seconds that you can use when you want.[/color]
I'm assuming the 5 seconds isn't a countdown to chicken explosion. I'm thinking that you have up to 5 seconds before you auto-explode (or change back...but auto-exploding makes more sense). Otherwise, you could turn into the chicken and in the 2 seconds it takes to run into the mob of baddies you press "explode."