It’s (not?) rocket science
Belfree WRS sensors contain a microprocessor, a 2.4Ghz digital radio and an Inertial Measurement Unit or IMU.
The IMU measures minute variations in linear and rotational acceleration. If we read the data from the IMU frequently enough and add it all together we can track the rotation angle of the bell. We compare this angle with the one we measured during the initial calibration phase. When these angles are equal the bell is, at that instant, at the bottom-most point of its rotation (we call this Bottom Dead Centre or BDC).
This process is called dead reckoning and is used for nautical, aerial and spatial navigation. (So it is rocket science!)
A signal is sent by the radio from the bell tower to the USB dongle in the ringing room whenever the bell passes through BDC. This signal is interpreted by whichever simulator software you are running to determine which bell has been rung.