Any card that has an ambush symbol allows you to knock out a competitor's Pawn and return it to its Ship. They're also easy to spot because the have a red pawn symbol on them.
Ambushing is done by moving your Pawn into an occupied square. Perform the movement on the card as normal, but move into a competitor's space.
The Sling allows you to throw a snowball in a vertical or horizontal direction from your Pawn and Ambush from a distance without moving.
You cannot sling through Pawns. Other things like cracks, Hole in the Ice, and the North Pole do not block the Sling.
The Pawn that was just ambushed gets sent back to its Ship.
The player who performed the ambush reveals the top Ice Mask tile from the corner of the board.
Base Camp squares are safe areas. You cannot ambush in, from, or to a Base Camp square.