[Classical Chinese] Would you please help with the phrase below? It is quoted in the book I am reading and I have no idea of what it means.?

提綱而衆目張,振領而羣毛理
This is an ancient term, its original meaning is as follows.
提: hold.
綱: the main rope of a net.
而:then.
衆:all.
目:mesh holes.
張: open.
振:lift up.
領:collar.
而:then.
羣:all.
毛:wool, leather.
理: (things are) in order.
Now days, the above proverb is simplified as ”提綱挈領”, which means to hold the main issues, to sum up the matters, and is commonly used in formal writing. Here, 挈has a same meaning as 振.