Nitinol belongs to a class of materials called Shape Memory Alloys (SMA). SMA's have interesting mechanical properties. Flexinol which is a trade name for Nitinol actuator wire, has been trained to contract when heated, which is the opposite of what standard metals do when heated which is expand. Not only does this alloy contract, but does so with a 100 times greater thermal movement (expansion-contraction) than standard metals.
Another interesting property of SMA's is called the Shaped Memory Effect (SME). The alloy can be made to remember a particular shape. Once a shape has been remembered, the alloy may be bent out of shape. Then returned to its original shape by heating the alloy about it transition temperature. This SME property is used in a toy called Nitinol Memory Wire, see figure 2.
You can bend the Nitinol Memory Wire toy into any shape you want, then place it in hot water and the wire quickly whips itself back into shape. It is interesting to watch this wire almost magically unfold and uncoil itself back into its original shape. The Nitinol Memory toy is made of nitinol wire with a low transition temperature (the temperature of hot water). The force generated when the wire is reverting is surprisingly strong. One square inch of Nitinol material generates a shape returning force of + 30,000 PSI.