OK the idea is that in autism the senses are buggered. Roughly the severity of autism might correlate with this. The idea is that the sensory overload suffered by a severely autistic child leads to them 'switching off' a sense in early life so they can make sense of the world. So a child who comes to rely on vision is good at imitating (hence can imitate motor skills then speech). A child who switches off vision because its too overwhelming loses the ability to imitate affecting motor skills then speech. This affects the ability to produce voluntary motor actions including speech. If they've 'switched on' hearing then they may have very good language skills but their motor problems prevent them from being able to demonstrate that (add in some Donna Williams type 'tics and fleas' as she calls them and voluntary behaviour is out the window).
This is adapted from the Strange Son book and is a theory put together mainly from talking to non-verbal people who can type.
It's only a theory, and I suspect a bit simplified but I think its interesting. DS1 has only been able to imitate for a year (and he has become more vocal in that time and has started to model behaviour on others since the imitation kicked in). For a long time he's been able to follow verbal instructions without cues (eg delivered with my back to him- just now for example I said; normal speed 'if you want a biscuit go and help yourself' and he did.) And it explains why some children when presented with a letterboard are found with intact spelling and language abilities.
Ds1 hasn't got that intact ability imo but I think he's been relying on smell and touch more than hearing. But it gives hope. If he's imitating perhaps he's managing his sensory problems better, so perhaps he'll improve further.
Fingers crossed anyway.
There's also some stuff about arousal patterns in autistic children which make it hard for them to learn from experience. All of this is in the Strange Son book - it's a bit of a tabloidy read for want of a better word, but it's very interesting and it does kind of tie the (limited so far) research together into a very simple easy to understand model.