https://www.youtube.com/watch?v=5c6mf8-RqOI@CA It works by sending out radio waves and receiving reflected radio waves from the object in motion. It calculates velocity from the Doppler effect (the frequency of the reflected radio wave shifts based on the speed of the object). It's the same thing that baseball coaches/scouts and the police use to clock speeds.
Since it works on a reflected wave and the object is moving and can change velocity, the reflected waves it receives will have a frequency that isn't constant. So, the value it reads out (displays on the screen) is the highest speed that it "sees".
Compared to one baseball scouts use a lot ($1500), it is within ±1 mph ( in the video I linked above).
Timpa also has one, and claimed he's used it to get speeds from objects as small as a metal BB. (I think it being metal helps increase the radio wave reflection).
https://youtu.be/4FgcysbplWAIn related news, I saw a really neat video of a guy who made a simple version that would emit sound based on the speed of the object (much cheaper than the unit I bought). But from what I can tell, it would be really easy to use the Fourier spectrum from the oscilloscope to give a max velocity too.
https://youtu.be/Kzsh59TM4MY