Wingsuit FAP is an app that helps wingsuit pilots train for Fast, Accelerated Proximity flight mode. It uses accelerometer to measure the percentage of reduction in apparent gravity, which is a gauge of the linear acceleration along a constant glide slope.
The idea of Fast, Accelerated Proximity is to always start the line at safe speed (for safe, confident disconnect at any time) and exit it at even faster speed, by performing a proactive gradual change in flight configuration in such a way as to accelerate along the line.
The app is meant to be used in skydiving environment only. Place the phone in a tight pocket so that it is pressed firmly against your body. Fly at constant (relative to air mass) glide ratio while changing your flight configuration to accelerate along the line. Listen to audio feedback using headphones - the higher the percentage of apparent gravity reduction, the faster your acceleration is. Try keeping the acceleration constant along the whole length of the line and disconnect when your speed reaches absolute maximum you can achieve for a given slope angle.