I see that there are various interpretations on what is / are the "nine crooked pearl(s)" as referred to in "Tai Ji Quan Xing Gong Xin Jie".
One interpretation is that there are nine different energy generating points (Nine Crooked Pearls (plural)) in our body and we coordinate them together to generate "chi" power.
Some say that there are nine joint obstacles and we need to break through each of them and the method use is the "Nine Crooked Pearls" (plural).
Some say that it is some sort of "Chin Na" techniques.
I do not wish to comment on these views.
The "Nine Crooked Pearl" came from an old Chinese story - Confucius was challenged by a lady as to the way to pass a string through a pearl with complex routes inside - the Nine Crooked Pearl (singular). The answer was to tie a string to an ant and used honey to lure the ant so that it led the string through the routes inside the Nine Crooked Pearl.
The classic simply said that we have to direct our energy, through the "chi" channels / routes within our body, to each and every part of our body - that's it.