This sensor can detect flexing or bending in one direction. They were popularized by being used in the Nintendo PowerGlove as a gaming interface.

These sensors are easy to use, they are basically resistors that change value based on how much their flexed. If they're unflexed, the resistance is about ~25KΩ. When flexed all the way the resistance rises to ~100KΩ. They're pretty similar to FSRs so following this tutorial will get you started.

The bottom part of the sensor (where the pins are crimped on) is very delicate so make sure to have strain relief - such as clamping or gluing that part so as not to rip out the contacts!

Note that the sensor can only measure bending in one direction. Bending the other way doesn't do anything.