It tries to describe what you need and when you need it most, like when you have the light on when it is burning high, you hardly need it, but you need it when it is burning low, because it is dark, but you hardly need it when it is light outside. " Only need the light when it is burning low ", is what I am describing.
Another example is: only miss the sun when it starts to snow. This means miss the summer when it is winter because it is cold outside in winter, but hot outside in summer to defrost us. Same thing backwards, only miss the winter when it is summer because it is hot outside in summer, and it is cold outside it winter to cool us down. So think of summer when it is winter, and think of winter when it is summer.