> Wow, I just done this and my whole upper back started cracking
You have had bad posture for a long time.
> How does this fix my pain long-term?
Doing it daily may or may not fix the problem. It surely has fixed my upper back problems. I do it and just few other back stretches. I also go running frequently.
The way they do it in the video is not perfect in my opinion. I could not find the way I do it anywhere. So here is instruction:
Take the same posture but instead of letting knee stay up and hand on the floor, push knee to the floor and let the hand hang palm upward above the floor (it really stretches your pec). Gradually you should be flexible enough to have both knee and hand on the floor.
>or shorten the back muscles that are stretched?
Short time stretching can't shorten muscles. Muscles shorten when they are never stretched.
Do the following test: Stand straight look forward. Have a pencil in your fist. When you are standing relaxed, look at where the pencil points. If it points inward, your shoulders are turned inward. Pencil should point directly forward.
How does this fix my pain long-term?
> your pectoral muscles are being shortened
Does this move stretch the pectoral muscles? or shorten the back muscles that are stretched?