The knight, because it must land after being moved, kills whatever it lands on. This means that it is the only piece that can kill a moving piece, even when it was moved after the enemy's piece. In the tactics video, white is in a desperate sitution, so he puts his king in danger to tempt the queen to move. Then he must time the knight just right to land on the d3 right after the queen gets there. Too early, and the knight will be swept, too late, and the queen will escape. Always be on gaurd for knights, and their tricksy ways.