Toph & Hama discovering a high level of bending technique while being trapped.
Metalbending: Toph discovered metalbending when she was captured by Xin Fu and Master Yu. Her seismic sense allowed her to sense the earth in the metal and be able to bend it. All Toph felt was pressure and pain inside that tiny metal cell.
Bloodbending: Hama was captured from her home, she was trapped in a metal cell. The guards were careful on not letting any drop of water to the waterbenders, they would even bind their hands when they need to drink. Hama realized every time that where there is life, there is water. She used the rats during the full moon and practiced this for many years, until she was ready for the men.
Both also freed themselves using their discovered bending technique.