Haven't you seen those commercials with the eyeglasses you can basically bend and wad up, then they spring back to their original shape? What is the material they are made of? It's called NiTi or Nickel Titanium Alloy. It's kind of a weird metal alloy that likes to retain it's shape even after deformation. This has been a boon for orthodintics.
Think about it. If you have a straight wire, make it "crooked" or otherwise deform or bend it to the shape of your not so straight teeth, then the wire wants to be stright again slowly moving back to it's original position bringing the teeth along for the ride. That is slow constant pressure over time. That's what moves teeth.*
There is even a varient that is soft when cold. It's limp like wet spaghetti when cold. It is then easily placed on the teeth. As the material warms up in the mouth, it wants to be in it's original position. Weird Science!
More NITi info here from 3mUnitek.
*And more braces info here: When is the best time to start Braces?.