Learn English – The meaning of “As if on cue”

A 'cue' originally, is a 'sign or symbol for an actor to go on stage'.

https://www.google.com.sg/search?q=cue+definition&ie=UTF-8&oe=UTF-8&hl=en-sg&client=safari

It lets the actor know when to walk on. On TV or radio it might be a verbal or visual countdown. In movies it might be '3, 2, 1 and - action!

So it's a sign that lets everyone know what is going on, when.

If you 'cue someone in' then you are the one that 'counts them in' so you say: '3, 2, 1 and - action!'

This meaning of 'cue' has been broadened to include other kinds of 'cues' - or 'signs' that we get from the world around us - about 'what is going on - and when'.

For example:

Your dog, showing up with his lead in his mouth - is giving you a cue - for 'walkies'! Or, he is 'cuing you in' to the fact that it's 'walkies time!'

Your boss, sitting you down for a chat about how punctuality is important is subtly, 'cueing you in' to the fact that you'd better shape up - or you may risk losing your job.

So 'cues' these days, may be verbal, eg: 'please take a seat here, Miss Thomas' or 'non-verbal, such as from 'body language' eg: 'the way he shrugged his shoulders made me feel he didn't care'.

The examples you give are talking about the sensitivity we may have - or may not have - to such verbal and non-verbal 'cues'.

Where we are sensitive to others and to our environment - we can say we are 'cued in' - eg: 'my therapist cued in to me really easily - he understood what I meant, straight away'.

Where we are insensitive to others or to our environment - we can say we are 'not cued in' eg: Friend 'Mary just got the sack!' Other friend: 'Oh no! really? - well, she was not very cued in to her boss - he warned her several times about lateness, but she just didn't seem to 'get it!'

She 'missed her cue' - and now she is out of the movie.

The writing in the examples you give, is a bit 'jargony', if you don't mind me saying. 'Cue in to the infant's social overload' sounds a bit odd - I am guessing they mean 'respond to the level of social overload the child is feeling' (presumably, by not creating more (overload) for that child by ...adding only one stimulus at a time.

'Cue in to the body's signals' means: 'pay attention to...' signs from the body...

I ask my listeners to cue in to specific areas' means 'I ask my listeners to focus on or connect with specific areas...'

Another way of saying to 'cue into' is often 'to connect with' or 'to become aware of'. It might help to substitute those words, to understand what is (trying!) to be said.