Pretty much the idea is to "season" the fish. The fish wont die if you fast them. The theory here is to have the fish use all it's reserve fat through the winter, the conformation can get better during the growing season next year. This practice was made popular by Minoru Mano "Dainichi".
Thing is as hobbyist we feed heavily during the summer months and the fish carries all that fat it consumed during summer into winter. Me personally I still feed once aday or basis my feeding on how active my fish are. There are days I feed once and there are days I dont feed at all.