Mirror, mirror on the wall.
When a Practitioner looks into a mirror all he sees is a reflection of himself. When he looks into his heart all he sees is any activity of the heart that corresponds to his gazing at it. The Practitioner thus mindful, will only ever know the fading of correspondingly previous activities or concerns.
However, given that the basic nature of the Citta is in 'just knowing', any Practitioner who simply gazes at the heart, watching it's functions, will gradually notice profound changes as the Kilesas (the egotistical - neurotic activities that he is, thus far, unaware of) die down to this basic level of 'just knowing': the analogy with the mirror then breaks down completely, for in this case the 'reflection' disappears.
Though, perhaps more unexpectedly, the reaction to this is one of immediate recognition: a coming home. The Citta finally realising it's own true nature.
However, this does require that the Kilesas are entirely dormant, if only briefly in a 'Kammic window of opportunity', for (both good and bad) their response will give rise to an unexpected experience of some disquiet for the Practitioner.
Hence the need for a conventional or intellectual understanding prior to any practice.