After your fermentation as completed, you take the fermenter and drop temps to as close as freezing. This promotes yeast and trub to settle. Even more beneficial is that if the beer develops chill haze adding fining agents (gelatin and others) at this point will help remove those proteins that cause it.
Adding gelatin at any degree will work but just to a much lesser extent. The idea is to add the gelatin and getting it to coagulate so the proteins attach to the gelatin and settle out. The bigger the particle the quicker it will settle out.
