This article by British baritone and teacher Neil Howlett is one of the best on this subject that I have read. It explains why it is a useful exercise, how it takes care of the “placement” issue, and then he explains how it can be taught, from the beginning of voice study. If I had to recommend one article to teachers about a pillar of voice teaching, this is my top choice these days.
Teachers and singers who find the classic definition of the messa di voce daunting – this can be approached in small, simple steps. The acquisition of the long and extreme version throughout the range is a multi-year project, but benefits can be gotten immediately when it is approached in the manner advocated in Mr. Howlett’s article.