What is slyness-concealment?  It is to perpetuate a state of unresolvedness [kha na ma tho ba] because of its association with dullness and stubborness [gti mug] when one is urged towards something positive.  Slyness-concealment has the function of preventing one from making a clean break with it and feeling relieved.
It is the intention to cover up one's evil through stubbornness when one's spiritual friends or others, in their desire to help one, raise the point of one's evil.

By concealing one's evil and not admitting it, evil, however small, will grow bigger and bigger and will provide a cause for not allowing one to make a clean break with it and feel relieved.  It will throw one into evil forms of existence later one.