The biggest mod for the auto allows it to downshift quicker (for climbing etc) to allow you to keep the wheel speed up. The other end of the mod allows you to adjust the engagement point - (not an accurate example): lets say stock it engages at 1000 RPMs, but you can mod it to engage at 3,000. I was surprised they were so customizable as well - and admitedly, I have a lot to learn about them still. Its pretty cool if you dont like the engagement you went with, you just swap to a different spring and it changes things up, with minmal work.