Gravity inversion works, very simple by stretching (decompressing) the spine. Most back pain is caused by either a herniated disc (I have this...L4/L5

) or a nerve being pinched between two vertebrae.
When you're inverted, the vertebrae are pulled away from whatever they are compressing, giving temporary relief. How long "your" relief lasts is entirely subjective and varies widely from case to case.
I've been going to a Chiropractor for almost two years now. I get adjusted every two weeks and usually get electro-stim/heat therapy on my lumbro-sacral area. Eventually, I will need surgery, but I am not messing with that until I literally cannot get out of bed.