my question on dilation,

All it is stretch of muscle. ,does it not snap back tight again after few days naturally.

What is the mechanism here ? Is it life time?

If that's the case , any one doing anal sex should have good stretch.I see lot of folks get it still.

I'm just trying understand better

I think the idea is to do it regularly so that the stretching of the muscles remains consistent, and BMs will help with the stretching too. There are often trigger points in the pelvic floor muscles that once massaged out should stop being an issue. Some people (such as me), get Botox to help get more long lasting results with dilation.