This hive above as you can see never really made it up into the second box. Being that this hive uses medium frames, a single box is not much. Normally I wouldn't be so concerned about a small hive like this, but I want to raise medium nucs this year. And while I don't need a medium hive to make queens for medium nucs, I do need brood on medium frames which means I need some medium hives to survive. So I took out half a dozen frames or so and laid a paper towel across the space and put a couple pounds of sugar in there. So you can see the result.
The time to feed is naturally when all other options are off the table. It's early or mid winter and you can't feed syrup. Feeding is a last ditch effort to save a hive, not to prop it up, but to maintain its utility. These hives that I fed will not be used for honey production or breeding. For the most part, they will be split up to donate their brood to new hives which will be headed by queens bred from the best. If they are lucky, the queen and some brood will be placed in a nuc to draw comb for the spring and then probably killed either by summer or by being combined with some stronger hive. Much depends on each hive's spring performance. A solid spring can make up a little ground from a weak fall and winter, especially for a yearling hive.