One of the banes of watching an IndyCar Series race the last two years or so was the dreaded fuel knob. This allowed drivers to adjust the fuel mixtures of their cars, allowing to run rich or lean if they wanted to save fuel. This led to drivers "making fuel" 30 laps into a 250 lap race, which made things quite boring.
Well, that's not a problem anymore, as Honda has ripped out the knobs. Drivers can still save fuel, but they'll still have to do it the old fashioned way, with their feet. This change has the added effect of allowing the push-to-pass button to become more powerful. Previously, a full rich car would only get an extra 5 hp with the button.