With a single move you’re doing ONE quick move to beat the goalie. If the goalie is caught flat footed and you’re quicker than they are, you score.
A good way to freeze the goalie is to move the puck first to one side, pause for a second to have them line up with the puck and think you’re going to shoot, then move quickly to the other side. You can also toss in a quick fake first for added deception.