An example might help - suppose the coach notices the client always refers to their boss with an ironic laugh. The coaches' intervention then might be something like 'I notice you laugh every time you mention your boss, could you say a little more about what is going on for you...?' Equally, the coach might pick up a sense of unease when talking with a client which could be shared as 'I'm feeling a little uneasy as we talk about ...., is that the same for you?'
Key to the use of this skill is firstly, selectivity and secondly, tentativeness. I would only use my perceptions of the client if I thought they were relevant to the issues at hand and only if I had more than one example to share -- i.e.. a pattern. I would also make sure I offered them in a tentative, provisional way rather than as a judgement. This can involve taking a risk, but I have found that this sort of intervention is often invaluable and will lead our conversations into some rich and unusual places.