Jhanic concentration and feeling

Could anyone share your experience and knowledge? People who have attained strong concentration have the ability to read other people's mind. My question is, what is the 'knowing'? Do they read other people based on what they feel? For example, one person concentrates his mind on a person, when his mind is calm, other people's mind will be felt and reflected in his feeling. Is this correct? How do they know, based on what? Any reply would be great, thanks.