I'm sure someone will have a link.
Basically, children with social communication issues don't see the point of any sort of communication, including speech or signing. They can have the tendancy to believe that everyone already knows what they are thinking. They see no need to tell you they are thirsty, you should already know, etc. This is called Theory of Mind.
Also they can find abstract concepts, such as feelings etc, much harder to understand than concrete ones that they can see and touch, like a biscuit or a cat etc.
Words can seem much too abstract to them. How can a sound that is fleeting and invisible signify something they can see and touch? Similarly sign language can be too abstract for some children (better than words though, as they can see the sign at least)
That's when PECS can be useful. It stands for picture exchange communication system. You have a picture of an object on a small card. We started with something DS wanted. A small piece of a biscuit. You needed 2 adults to start it off. A card with a picture of a biscuit (starts with maybe a photo, can eventually be more of a symbol as they get the hang of it) is placed on the table. One adult sits with the child on one side, the other adult has the biscuits on the other side, out of reach. The child hopefully wants the biscuit and does his/her normal thing to get it. The adult with the child shows them the picture of the biscuit, takes their hand and helps them pick up the card. They say, "Biscuit, I want a biscuit". they 'help' the child gives the card to the other adult, who takes it, and says "Biscuit. I want a biscuit" and gives the child a biscuit. (Small bit!)
This is repeated ad nauseum until the child gives the card on his/her own. Reward is a small bit of biscuit. The hope is eventually they say themselves "I want a biscuit"
You use the words they should use, don't confuse by saying "You want a biscuit" as they will then repeat that.
You go on to making choices, a picture of a biscuit and a crisp. They get to choose, they have what they have chosen. You expand to include choices of games or activities.
The point is, it is communication, even if very basic. The picture signifies the object. It is concrete, they can hold it. You back it up with words, but you don't expect them to speak at first. Never withhold the reward if they give you the card, don't expect words. Words may come later.
The frustration that was alleviated, my DS could be in control to a small extent. We expanded to include more symbols. "I want" PEC followed by a game PEC, or the park etc.
This system taught my DS the purpose of communication. We stopped using it after about 6 months. He went on to start asking for things, usually still after I gave him the words in the choice question, but he was speaking, within months. I could say, "Do you want milk or juice?" He'd answer "I want juice".
If I'd said, as I used to, "Are you thirsty? What do you want to drink?" He didn't have the words to answer. He needed to hear the choice. He still does a little.
All this was when he was 3.5. He's now 11 and will be going to MS Secondary in Sept!
What an epic! If you've got to the end, well done. 