Time to review the Laws of Torque Absorption there Professor AP!
I'm with Elroy - it stands to reason that if the extension twists it IS ABSORBING torque - MAGIC? Only if you have no clue. Sheesh.
Let me use an analogy...
You just caught a monster Catfish in the local creek. So, after unhooking it you decide to weigh it with your fish scale. But, it just so happens that your fish scale has a 10' rope attached between the scale and the hook you hang the fish from. When the fish is hung from the rope the rope stretches to some degree relative to the weight of the fish. However, the system eventually comes to equilibrium and ceases to displace. At this point, you take your reading.
Because the rope has stretched, does the scale indicate the fish to be lighter than it really is, or would otherwise be indicated if the rope was not between the scale and the fish?
Therein lies the answer to the "absorption of torque" by an extension.
If the extension twists, it is absorbing work, not torque. More specifically, it is changing work into other forms of energy according to the law of conservation of energy. Work is equal to the force (torque) times the distance (angular displacement).
Changing work into other forms of energy is how "torque sticks" used with impact guns work. They change varying degrees of work produced by the impact gun into heat, noise, and torsional vibration between pulses of the impact hammer. The impact gun is only capable of imparting a small change in angular displacement with each hit of the hammer(s) before it must release and reset itself for the next blow.
This limited range of motion allows the torque stick to limit the maximum static torque though torsion of the extension. Essentially, the torque stick acts as a torsion spring. By varying the stiffness of the extension, the slope of the stress/strain curve of the extension changes. It is the slope of this curve and the length of the extension that determines how fast torque builds relative to the finite angular displacement available from the impact gun. Strong springs have a high slope and build tension rapidly when pulled. Weak springs have a low slope and build tension slowly when pulled. In all cases, longer springs of the same section have a lower slope due to their greater length.
Since torque wrenches are generally not used over a finite angular displacement, the torsion of the extension is immaterial. If proper technique was used and the wrench clicks, the correct torque was applied.
