If we think about how a game of football evolves, we can see a player is presnted by a series problems, reactions to those problems and solutions to those problems (problem-reaction-solution).
As a player we are constantly observing our surroundings and the way our surroundings appear to us directly influences our actions. If we see a problem, we must react, to create a solution to that problem. If we do not, we lose.
The ability of player to have fast and positive reactions to his surrounds requires the player to have an excellent grasp of all the insight to the game.
The cue to take the action of actually dropping off, is the execution of the pass from the first attacker. However, observing that all the other components are in place for you to do so, like your second defender is ready to put pressure on the ball, should have been considered also.
Maybe your team plan is to play an offside trap. If this is the case, then you have a new set of cues and actions.
If you were the first defender and the attacker that you are marking passes the ball, this might be your cue to check your teams defensive line as you drop off. If the attacking player is running into an offside position and you also observe that the ball is about to played forward by the attacking team to the player you are marking, then you would “step up” to leave them offside.
We can see this example in the video above. Holland are defending a free kick. The whole defence is aware of what they are about to do.
As the attacker takes the free kick, his approach to the ball is the defensive teams cue to all step up and leave their opponents offside.
Conversely you can consider a scenario from the attackers point of view in the video below.
Totti and Cassano show their understanding of defenders poor positional space and their inability to drop off quickly enough, as their cue to action their passing.
They attack at pace and make full use of the space that is left behind the defenders that are pulled out of position. First one and then the other take turns at moving into the space left by the pressuring/first defender.
Their movement, understanding of the situation, understanding of each others intentions and their speed opens up a great opportunity to score.
Again, it is the players insight of the game that allows them to take advantage of the situation.