The tension wrench, also known as a torsion wrench, is used to apply torque to the plug of a lock in order to hold any picked pins in place.
Once all pins are picked, the tension wrench is then used to turn the plug and open the lock.
It is typically shaped like a letter "L", although the vertical part of the letter is elongated in comparison to the horizontal part.
Uklockpickers would strongly reccomend the flat bar tension tools from sparrows an essential aid to your kit
Remember tension is the key to opening all locks