Wikipedia informs us (translated to English by yours truly):
"To gnussa is a variation of or addition to the couples dance Foxtrot. When dancing the foxtrot most couples do this with their heads next to each other, eyes straight ahead, cheek to cheek. To "gnussa" the couple instead support their foreheads lightly against each other. As their feet move the couple roll their foreheads smoothly against each other. The couple always look in the same direction and always in the direction in which they are moving. If the couple has a big difference in height the shorter partner can instead support their forehead against the taller partners lips or chin.
To gnussa probably originated in the area around Sundsvall, Sweden, and arose early in the 1990s. From there it spread to the rest of Sweden, with growing popularity."
Let's let Jonathan and Lisa demonstrate the moves for you.