You never need to start, stop, open or close your app to accurately capture your driving.
To be a bit more scientific, the app is "sleeping" in the background waiting for driving-like behavior (it does this to minimize data and battery consumption when you're not driving). When it sees movement that looks like driving (e.g. speed, location), the app begins recording. The data from each trip is processed so that we can show you things like the route you took, the distance and time you drove, and scores on things like distracted driving, high speed, hard braking and time of day.