Seeing INwards, rather than outwards. Where there is a connection there, where the matter had been pondered before, than that is insight. At times “suddenly a jump appears and the understanding appears” Then we are on the fast track. But as to Dhamma, for many it is not so.
There is a sutta where five followers were in attendance when the Buddha was delivering a discourse. One looked up at the sky, another did something else, yet another something different; only one listened. The Buddha later gave the reasons(s) why each did what. The last one, the one who listened attentively, had heard and attended to the Dhamma before (in previous lives).