A vibrating wire sensor measures the opening of a joint from the stretch of a wire being made to vibrate at acoustical frequency.
Since the wire is made of an elastic metal, this type of sensor can be used to measure pulling forces within a certain range.
The applied external force changes the tension on the wire, which changes the frequency.
The load sensor has an integrated electronic system to activate the vibrating wire and measure the frequency.
This can be compared to a guitar: plucking a string creates vibration and sound.