The accelerometer’s z sensing axis aligns with gravity when the micro:bit is held level with its LED display facing down.  In that position, accelerometer.get_z() returns a measurement in the -1024 neighborhood. 

Held vertically, it returns zero since the z-axis is perpendicular to gravit y— it doesn’t line up at all.  If held vertically, rotation angle won’t matter to the z-axis, it still returns zero.  Held with the LED display facing up, the z-axis is aligned with gravity again but pointing the opposite direction.  Because of that, the accelerometer.get_z() function returns a value near -1024.

As for how the script test-z-axis-tilt works, it is basically the z-axis version of test_tilt_intro.  The main difference is that:

    x = accelerometer.get_x()
    y = accelerometer.get_y()
    
    print("x =", x, ", y =", y)

…has been replaced with

    z = accelerometer.get_z()
    
    print("z =", z)