The combination of heat and agitation are needed to shrink fabric (especially wool). Heat makes the normally kinky, twisty, grippy fibers more flexible and looser. Agitation allows the loose fibers to reach other fibers farther away and grab onto them as they cool, pulling them closer, thus shrinking the fabric.
Heat and very careful stretching can somewhat restore the original size, but restoring the shape is harder.
For the record, agitation is not necessary, although it may increase the shrinkage. I have often successfully shrunk smaller pieces of fabric by simply wetting it and ironing it dry. I watch the fabric shrink before my eyes, just as the fabric gets dry. I do this with cottons, linens and rayons.
The combination of heat and agitation are needed to shrink fabric (especially wool). Heat makes the normally kinky, twisty, grippy fibers more flexible and looser. Agitation allows the loose fibers to reach other fibers farther away and grab onto them as they cool, pulling them closer, thus shrinking the fabric.
Heat and very careful stretching can somewhat restore the original size, but restoring the shape is harder.
For the record, agitation is not necessary, although it may increase the shrinkage. I have often successfully shrunk smaller pieces of fabric by simply wetting it and ironing it dry. I watch the fabric shrink before my eyes, just as the fabric gets dry. I do this with cottons, linens and rayons.