It means: "When the dark months come the nipples get hard."
It's a Fin saying they have as a joke about how cold it is when they have barely any light for months.
The saying comes from fishing. First we give them some free fishing line, then the bait supposedly catches onto a rug; when we start to spin the reel, the rug or "matto" gets pulled from underneath their feet. You can see that in a lot of ence's games with *coughs* losing all the pistols they have ever played lol
First we give them some leeway then we pull the rug from underneath their feet