When you make something straight, you unbend it. If your knees are stiff, it can be hard to unbend them after sitting cross-legged on the floor for a while.
To straighten something bent or twisted is to unbend it. You can finally unbend your arm once you get that cast off! And when you relax or loosen up, you figuratively unbend: "It'll be nice to take it easy this weekend, and finally unbend after a long, busy week." The original meaning of this verb was "relax a bow by unstringing it," from the word bend and its earliest definition, "bend a bow."