The diagram below shows the typical way capacitive touch screens are implemented. One of the lines is driven to a certain voltage while another line parallel to it is grounded. When a finger (which is also grounded) comes near the lines, it takes away some of the electric field between the two lines, thus reducing the capacitance between the lines. This leads to a change in stored voltage, which can be used to detect a touch.

We implemented a system that is similar to this, but instead of putting the lines next to each other, we stacked them up at a particular point and measured the capacitance at that point. Since we stacked the lines instead of lying them next to each other, bringing our finger near the lines didn’t change the capacitance between the lines. Instead, the touch added additional capacitance between the top line and our finger (ground), resulting in two capacitors in parallel. We measured this increase in capacitance quite easily (parallel capacitances add up).
Here’s a short video of the touchpad we created (please turn on your volume, to listen to the sound when a touch is detected by the screen).