To use the "scale," you simply place one or more fingers on the screen. The interface will instantly generate a numerical value representing the force or area, alongside a real-time graph.
If you don’t calibrate correctly with a known weight, your results will be meaningless. A dirty screen or uneven surface will also ruin accuracy. Touchscale.ml.mk
If you are looking for formal research into the technology behind such apps, these papers explore similar 3D touch force estimation and capacitive sensing: TouchScale : Weigh things on your iPhone! To use the "scale," you simply place one