The key clue that the wall pass is on is the immediate and hard run by A after he passes the ball. The key to start this run is when the dribbler takes the ball laterally towards the overlapping player. The overlapper, essentially runs into the space vacated by the dribbler. Here the overlap is when a back player runs past a forward player. This is most often a 3rd attacker run and the key is when the forward player moves and leaves space open to run into.
Drops are useful in two circumstances.
First, when there is heavy pressure on ball where a more positive pass cannot be made. Secondly, to reverse the ball from one side of the field to the other and to unbalance a defense. There should always be a supporting player in a drop position. Whenever a positive play cannot not be made, the drop away from pressure is probably the next best option. There are certainly special times for vertical passing, but on the whole, make your passes at angles.