What characteristic defines a torsion spring?

A torsion spring is uniquely designed to operate by twisting. This is the essential characteristic that defines it: it stores mechanical energy when it is subjected to a twist or torsional force. As the spring is twisted around its axis, it undergoes deformation, allowing it to store energy. When released, the spring will return to its original position by unwinding, effectively releasing the stored energy.

The other options describe different behaviors or functions not applicable to torsion springs. For instance, springs that store energy when compressed would be classified as compression springs, which operate differently than torsion springs. The rapid release of energy without storage pertains to different types of mechanisms altogether. Additionally, the specification of functioning solely for tension applications would relate to tension springs, which are designed to handle pulling forces but do not apply to the twisting action characteristic of torsion springs.