The weak side defense will typically have to pick up the attacker that is driving the net. They should be slightly staggered behind the strong side defense and therefore they are the last player back.
Players are always taught to backcheck hard, in fact, more often then not players that don't backcheck hard are quickly escorted to the end of the bench for a while (depending on the coach). Backchecking hard is only half of the equation.
Teaching players to backcheck effectively is just as important.
Not all situations are the same and therefore players should be taught to recognize and react to the situation at hand. There are also basic fundamentals of backchecking that should be taught regardless of the situation. Incorporating good backchecking drills into practice is important so players become familiar with situations and can react instinctively instead of having to think about it is a game.