When flying with the elytra wings and this mod, you can press the assigned (default "R") key to enable "Auto Flight". The auto-flight will modify your pitch (the up-down direction you are looking) to cycle between going down and gaining speed, and going up and gaining alitude. If you start from a high enough point, you will gain about 10 blocks of altitude per "cycle", and can thus fly for as long as your elytra wings have durability.
Pressing R when not flying with the wings will open the settings screen.
When flying, the mod will display a real time graph of your flight profile : 