to open all the way: to take something firmly shut and lay it open, so that what was closed off can now be entered, seen, or understood
This is the ordinary Greek verb for opening (ἀνοίγω) with a prefix on the front that pushes the action all the way through. The result is not simply opened but opened up: a womb that lets a firstborn through, ears that finally take in sound, a sky that parts, a mind that at last grasps what it had been staring at.
Seven of its eight appearances belong to Luke. In one scene at the close of his Gospel he uses it three times: the disciples' eyes are opened and they recognize Jesus, they remember how he had been opening the Scriptures to them on the road, and then he opens their minds to understand those Scriptures. The repetition is deliberate. Recognition, the text, and the understanding are all shut fast, and the same act unlocks all three.
From there the step to plain teaching is short. When Paul argues in the synagogue at Thessalonica, Luke says he was opening the Scriptures and laying the evidence out beside it. Most English versions translate that as "explaining", which is fair, but the picture underneath is of a text that was sealed and is now laid open.
In nearly every case the one who opens is God or Jesus, and the thing opened is a person or a text that could not open itself. Lydia in Philippi is listening, and the Lord opens her heart to pay attention. That pattern is part of the word's weight: the opening is something done to a person, not something the person manages alone.