Help with high level billiards puzzle

[Chicken Daddy]

I just got to the puzzle level with "reversal" and I'm missing something. If I could get an in depth explanation/video link on what to actually do with this step cause I seem to be fundamentally misunderstanding something. Haven't had any trouble up to this point with the puzzle

[Chicken Daddy]

this answers my confusion https://www.reddit.com/r/BluePrince/comments/1jxtas9/someone_make_this_math_work_for_me_please/