Since you've already put in the hours taking all your badass action footage, the app does everything else for you. Just drop in your video, upload a track from your music library, and let Shred get to work. The app's software can detect different visual cues from the video to create seamless action—matched perfectly to your tunes. And if there's anything you want to include manually, you can still jump in and take control.
The app itself is free, although Shred does charge a per video fee with different purchase options available. All in all, it's a small price to pay for perfectly edited montages of your adventures. Head here if you're a Mac user to download it yourself.