In the defensive zone, wingers have the responsibility of guarding the point-man (opposing team's defenseman) on their side of the ice. So the left winger guards the opposing right defenseman and the right winger guards the opposing left defenseman. The wingers typically patrol the area from the blue line to the half boards, to the high slot. When the puck is on the left wing side, the right winger should shift over to cover the high slot, and vice versa.
The key to playing solid defensive hockey as a winger is to stay between the man you're covering and your own net at all times, especially if you're playing man-to-man defense. Gap control is something that defensemen must learn to execute with precision, but all players must understand the importance of gap control as it pertains to their specific position, especially in the defensive zone. Wingers must always be within 1 to 2 sticks' length of the defenseman who they are guarding. Maintaining good gap control allows you to quickly cover passing lanes and block shots, quickly reacting to plays as they develop.