Understanding comes by the 18 principal insights:

1: Reflection on impermanence leaves the illusion of permanence,
2: Reflection on pain renounces the misapprehension of pleasure,
3: Reflection on no-self evaporates the imagination of a my-self,
4: Reflection on dispassion and disillusion abandons naive delight,
5: Reflection on disgust eradicates voracious and insatiable greed,
6: Reflection on ceasing desists from blind and initiating origination,
7: Reflection on relinquishment releases the lock of grasped clinging,
8: Reflection on fragility leaves the misconception of compact solidity,
9: Reflection on the breakup of all constructions shuts down accumulation,
10: Reflection on change corrects the deception of any lasting constancy,
11: Reflection on the signless eliminates the captivating appearance of a sign,
12: Reflection on the desireless diminishes the feverish urge within all craving,
13: Reflection on voidness erases the insistence on the self-deception: 'I am...',
14: Reflection on higher understanding uproots clinging to the fix idea of a core,
15: Reflection on direct knowledge & vision removes confused misinterpretation,
16: Reflection on danger extirpates false assumption of reliance on the unstable,
17: Reflection on resolute contemplation shelves the neglect of non-contemplation,
18: Reflection on turning away exterminates mistaken insistence on self-bondage...

More on Insight (Vipassanā):

Source: The Path of Purification: Visuddhimagga.
An excellent if not legendary translation by Bhikkhu Ñanamoli.
Written by 'the Great Explainer' Buddhaghosa in 5th century AC:

The 18 Principal Insights!