When did they add the 'overtake ist' HP boost in Indycar (IRL)?

The "overtake ist" was added before the Kentucky Race. It's been used in Kentucky, Mid-Ohio, Infineon and now Chicagoland. This feature has made IndyCar racing so much more exciting on ovals. On the road courses you haven't seen much of a difference, but the two oval races with the OA have been phenomonal! Both races won by the nose of a racecar. If they keep this up, the IRL will get much more popular.