This is the name given to an injury of the Ulnar Collateral Ligament of the thumb (the support on the index finger side of the joint, which stops the thumb being pulled outward). This often happens as a skier loses balance and hits the snow with the hand outstretched and the thumb away from the hand due to gripping the ski pole. As the thumb digs into the snow and the momentum of the skier keeps going, the ligament sprains or ruptures.
Not only does this injury seriously affect your ability to keep skiing, it can affect your ability to hold a hot chocolate, or a Gluhwein!
Treatment is as per any ligament injury, initial ice, splinting or taping, and reduction of activity (less pow pow, boo hoo!). Thankfully ice is in plentiful supply in the mountains, so just jam your mitt in the snow. There are some serious ongoing implications if the ligament is ruptured, causing instability and inability to grip, twist or use the thumb. Often in these cases surgical repair is necessary to restore function to the thumb, followed by bracing, rehabilitation and progression of exercises by a physiotherapist, OT, or hand specialist.