The measurement is triggered when the ball crosses the vertical line in the middle of the screen on the measurement device. Usually this line should be aligned with the net to ignore net balls.

If you want to measure net balls you need to move the measurement phone about 20cm towards the player. The vertical line is not on top of the net but parallel to the net on the side of your player. Now, the measurement will be triggered by balls that go into the net on that side. It will also measure balls that bounce back from the net.

If you want to go back to normal operation just move the phone back such that the vertical line aligns with the net.