If you somehow incorporated a 3d representation of each trick which you could rotate, zoom, and control the speed of, that would be incredible. Along with a text description and an audio-video description of each trick and you'd have an app I would pay some good money for. Also, having multiple explanations of things helps. Sometimes people don't quite understand what you mean.
Instructions on how to progress up to a trick is extremely helpful. For example, to progress up to a flatspin, you would first get backflips down and then slowly tuning them so that they are off axis. Maybe add a recommended checklist that one would go through which all help progress up to a certain trick, such as before attempting a nose butter triple cork, make sure you can nose butter double cork. And under nose butter double cork, it would recommend getting nose butter corks first, and before that corks, etc etc.
There are so many ways you could add little details to this as well, but that's up to you. The more you do so, the more likely someone will buy this app.