Grass may look "dormant" to us from above ground in winter, but underground growth continues as long as ground is not frozen. Even deciduous trees grow roots after leaf drop if ground is not frozen.
This year the leaf growth continued here until mid-November - really unusual, because temps did not fall. After a couple of hard frosts, generally, the leaf growth ceases or slows dramatically, and the grass plant switches to underground growth - roots and rhizomes - and to carbohydrate storage. A proper late fall application of fertilizer, after leaf growth has slowed helps this underground growth and storage. If properly done, it is unnecessary to fertilize next spring until Memorial day, thus reducing heavy spring leaf growth common with early fertilization.
If the ground happens to freeze shortly after your late fertilization, it is not wasted. It is frozen in place for use if the ground thaws again or until spring thaw.
see
http://www.agcom.purdue.edu/AgCom/Pubs/AY/AY-4.html