I’ve done it and still do it sometimes. Depending on how engorged the unit is, the impact will be more or less - with complete flaccid having the highest impact and complete erect making it impossible to create an impact. When doing it at an upward angle, my unit has a tendency to partially engorge and the exercise becomes self limiting with regards to any danger to the glans-to-CC connective tissue.
Not many people have tested it, and I’ve always used it as an adjunct to other routines so I can’t speak directly to its effectiveness.
That said, it can be overdone, and I think if it were taken to the absolute extreme, meaning it’s done for hours on end or days and weeks on end without rest and ignoring signs to stop - one could eventually rip their glans off. On the other end of the spectrum, if it’s done responsibly in short sets listening to PI’s, I’ve had no injury from it.
The original thread about it was started by a member named “ttrevor” if I remember correctly - a quick search will turn it up.