Set up a ranked Backlot Battle and have your boosting buddy join (online is dead so it's unlikely anyone else will join in). Have him drive in front of you and have him do a few stunts to build up a small string. Then you hit him from behind to steal his string. If you did it correctly, you should get a notification of the string theft. Repeat this six times in the same battle. You don't need to finish the battle so don't worry about crossing the finish line.