Unfortunately, the app has to be fully active to track anything. That means if has to be the foreground app and the screen has to be on, not locked. There's a "battery saver" mode that uses the gyroscope to attempt to detect if it's tilted down. When that works, it stops the 3d rendering and just shows a black screen with a logo. It helps the battery a bit, but the constant GPS and server pings are still a serious drain.
Also, you still have to touch it every few seconds to keep the phone from just going to sleep.
And there's a bug where the app stops responding to input when it "wakes up" from that mode.
Oh, and sometimes it gets the gyroscope backwards and you have to hold the phone upside down to access anything.
It's still a hot mess.