Is there a limit on speed? Can I attach a balloon, extremely long string, and parachute?
The app does compare distances neg-Y and pos-Y but I don't know if it checks the rate of ascent to be within some tolerance of throwing speed. You may have found the cheat code, if you can address the speed limit.
This app doesn't use GPS, it uses accelerometer. It is 100% legal in the US to operate a GPS while over CoCom/MTCR limits: above 1,000 knots (510 m/s) and/or at an altitude higher than 18,000 m (59,000 ft). The regulation is on the GPS manufacturer, who is required to output null/error/no GPS data if it detects it is above these limits (edit: unless they are licensed, which you can apply for if you're a defense contractor or in aerospace). It isn't like if a GPS device is traveling too high or fast, it is automatically a crime and bricks itself or alerts the authorities.
You still have to be careful to buy a GPS unit that isn't limited in altitude, though.
Do you have a source? I remember catching "800km/h" by a normal trekking Garmin device through the window of a passenger plane. Quick googling revealed many such stories.
> "Cheating" by throwing a phone from a tall building typically returns an error message.[3] The app's calculations keep track of how long the phone takes to rise and fall, and an error message is displayed if the distance fallen exceeds the length of the ascent.[2]