The act of 'doing' Right Concentration is what allows you to understand what it means to 'do' so well that you actually learn how to stop doing. That's the karma that puts an end to karma, the intention that allows you to understand intention until you finally get to the point where you can stop.
Mindfulness is where things start, but it can't do all the work. It's only one of the spices on your meditation shelf. This is why it's important to understand precisely what 'mindfulness' means, and how to supplement it with other skillful qualities in the mind.