Carrier oil equalization check valve

Hi friends,
I was reading carrier document, I saw something and I need your help to understanding it better.
Carrier says when installing two 06E semi-hermetic compressors in parallel, beside of jointing two side glasses as oil equalization line have to add oil equalization check valve and must be installed it in the rotor locking bolt I showed photos below.
Would it be possible tell me what is duty of this oil equalization check valve ?

Thanks.



Attachment 16085
Attachment 16086