This is almost short enough to paraphrase in its entirety. The Buddha states that state of mind changes quickly, so quickly he can't think of an analogy. He then describes that the mind is "luminous" but subject to defilement. The noble disciple knows this and therefore his mind develops.
The way it is stated, it seems the knowledge of the minds vulnerability is the sine qua non to mental development. Forewarned is forearmed, it what I get from this. Getting rid of defilement isn't the action, mental development is the action, lessening of defilement is the result.