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I am going to give this a shot. From my kindergarten level self taught Pali.
key words:
when one gains a knowledge that joy or dwelling in content of the infinite consciousness (empty of any permanent nature)- is a causal of a binding fetter, He practices insight wisdom in that knowledge.
My opinion, i.e. Buddha's meditation teachers when he was seeking enlightenment, those two meditation teachers did not see the binding force (fetters) caused by joy in (from) those meditative states so they did not gain any insight.
further simplify: when you know that joy in meditation is a binding, you practice insight from that knowledge.