Through the experiences of people who have tried it themselves, which have informally been collected by communities of people interested in this and other methods.

You are welcome to find out through direct experience whether it is effective for you.

If enough LWers try it, maybe some psychologist who reads LW will do a formal study, where one group meditates like this and another group does whatever. (Probably wouldn't get IRB approval, given what I wrote about stages two and three.)