Resistive sensors describe a class of sensors who work because of changes in their resistance. These resistance changes often come from a structural change in the materials. Resistive sensing works many times because electricity will always follow the shortest path from power to ground (+ to -). Thus, if a structural change changes the path electricity takes, the measure of resistance will change proportional to the length of the path.
I will explain the principles of resistive sensing using these pipe-cleaners as an example.