Simplify this!

What do the three types of immobilization have in common?

Lack of space
When a piece is under attack and has lack of space, it has a lack of time too. Imagine that your opponent was allowed to do a few moves in a row, while you had to pass and wait, then he could free himself.

Function
When a piece is immobile due to its duties it can't abandon, it is comparable to a lack of space. Imagine that your opponent was allowed to do a few moves in a row, while you had to pass and wait, then he could free himself.

Lack of time time
When two target pieces are placed in an unfortunate way, they can be both attacked at the same time with a duplo attack. Imagine that your opponent was allowed to do a few moves in a row, while you had to pass and wait, then he could free himself.

What these three times of immobilization have in common, is that they are temporary. They all lack the time to do all necessary actions. That is why special moves, aka duple function moves, are paramount. Both for the defense and the offense.

Sitting duck
I have been inaccurate about the sitting duck. A sitting duck doesn't equal the target in al situations. In situations of lack of space and duplo attacks, it does. But when a piece has two functions which are mutual exclusive, the target is often not the sitting duck, but the pieces it is protecting.
So the question is: how to exploit the sitting duck.

Diagram 1. Black to move

r3k3/p4p1p/4p1r1/2p1n3/4nN1Q/P1PqP2P/3B2P1/2R2R1K b q - 1 1
[solution]

points of pressure d2; f1; g3
lines of attack g-file; d-file; d3-f1-diagonal
function c1 defends f1; black Q defends e4
sitting duck d2

How to exploit the sitting duck?
I will update later. Comments are already welcome.

UPDATE
The white bishop is hanging. It is the sitting duck. The thinking starts with the question "how to exploit the sitting duck". There is no reason to think about an other idea. Unless you can prove that the bishop can't be taken.

A hanging piece is a common situation on CT. Is usually the result of an incorrect sacrifice. It is a typical example of a piece that is immobilized by lack of time. 

The normal reaction to a hanging piece is to take it. Cash it in. But there can be situations where the the capture should be postponed, since you first have to deal with a counter attack. As is the case in diagram 1. Notice how backward thinking diminishes the amount of things to think about.

We can't avoid to think from the point of view of white here. What are the points of pressure from whites perspective?
points of pressure e4; g6; e7; e5
lines of attack h4/d8; h-file
function Q - e4; Ne5 defends g6; h7 defends h8

1. ... Qxd2 2.Nxg6 is a triple function move. It attacks e4; e5 and threatens Qe7#

That indicates the three things black has to prevent before he can take the bishop. He can use as much moves as he needs, as long as he keeps the initiative. Since whites threat comprises mate on e7, keeping the initiative can't mean anything else than giving a check.

1. ... Ng3+ solves the first problem: it saves Ne4 for the moment with tempo. What is more, it chases the white king to the line of attack e3/g1. Which gives black a chance to gain a tempo while defending his knight on g3. Notice how important this is.

1. ... Ng3+ 2.Kg1 Qxd2 3.Nxg6 Qxe3+ the knight on g3 is save and the king has to move to h2. The rest can be easily calculated.

Backwards thinking minimizes the calculation by pruning the tree of analysis drastically. You don't have to consider a lot of options. The prize for this is that you must be very accurate about what a move actually accomplishes. Especially what effects it has on the initiative. 

The questions you have to ask your self are not rocket science. You have to know the standard scenario's how an attack or defense develops. We need these questions to pop up automatically. There are not that many scenario's or questions. We need to cultivate our sense for the initiative. And we have to cultivate accuracy. Two skills that aren't acquired overnight.