Many, many years ago, one of the old-time initiators told me that Mahesh had been quite excellent at the practise of generating heat. No one knew what it was called then, but the yogis would throw their dhotis into the cold stream and then put them on and generate heat. Apparently the first one to dry his dhoti was the winner.
Mahesh was often the winner.
Now, we know a little more about this.
for example, http://en.wikipedia.org/wiki/Tummo
The following video might be interesting
Lama Yeshe wrote a commentary on a medieval text on this topic: The Bliss of Inner Fire: Heart Practice of the Six Yogas of Naropa. What especially interests me is that Dr. Benson is using science to find out what is happening, as opposed to Mahesh's ego-maniacal we'll use science to prove it.
Mahesh was probably quite good at many of the siddhis. He impresses me as the sort who is quite good at most everything. Unfortunately, the less than convincing proclamations and batty behaviours in the years following being dumped by the Beatles lead one to understand just why Patañjali felt the siddhis were dangerous!
It is interesting that the Tibetans have taken one possible siddhi and completely transformed it into a means of realization. Personal guidance, exacting teaching and the guidance of someone who himself or herself has received the teaching from a lineage of teachers is far different from paying your money and, in effect, being told what to do by a robot.