only shiringan users, because when they run at full speed towards the enemy, they have to be able to read the enemies movements. if a person that can't read the enemies movements runs straight towards an enemy with chidori, they will be easily dodged/counterattacked/killed because they leave themselves open. I'm not sure if I'm explaining it right... but it's for shiringan users only. Why do you think Kakashi taught it to sasuke instead of Naruto? ^^;