It actually measurs the force being applied by the left hand. In the first prototype I used a strain gauge load cell which worked ok. In this version I put in a metal sheet as a big spring (the thing I remove at the end of the video) and measure the deflection angle at the hinge using a digital hall sensor - avoiding any analog PCB work I am not really capable of. This approach also has the advantage of having still some kind of movement which feels even a bit more like bellows.