They typically respond to both pull and push forces. What direction voltage change depends on how the two sense wire are wired to a needed following op-amp. Strain gauges are usually wired up in a Wheatstone Bridge configuration, two wires for the voltage source and two wires are the measurement output. Strain gauges typically output a very small voltage, say 0-5 millivolts for those examples posted if powered with +5vdc. So a instrumentation op-amp is required to raise the voltage range to something more usable for a arduino analog input pin. Such a op-amp with it's gain set to X1000 would allow a full measurement range of 0-5vdc. For your application where you are not planning of needing a calibrated range of measurements, but rather just a repeatable value when the tension is properly reached, you may not need full 0-5vdc input to the arduino.