What they're doing at Hengill is mentioned in the blog.
They're, "pumping carbonated water down into a very deep hole in the ground untill it all explodes?" (and from the same poster on the blog who I gather has some expertise in the area)
"They are pumping supercarbonated water down straight next to a thermal anomaly (magma-reservoir) so that it quite literally detonates into super-critical steam (dry steam) with the added explosivity of the CO2? Basically they are setting of hundreds of large mining blasts next to the core of a volcano. This is not anything close to as safe as normal fracking.
What they are doing is really crazy. They should be doing that experimentation a hell of a way from people. Krafla would have been a much better choice then Hengill for safety reasons.
Because whatever they are telling the icelandic people, they do not have a clue what they are doing. They are testing real dark arts pumping here."
A bit further on, this poster says:
"You know that this is a IDDP project? Ie, that they have drilled down next to the magma-reservoir, the same as they did at Kraflavirkjun.
So, they are at the fissure close to the reservoir, so all it takes is a bit to much fracturing and then they have built a man-made volcano "
I'm not sure of the details of why they're doing this but even to me, it sounds a monumentally stupid thing to do in an area as seismically and volcanically active as Iceland, especially right next to a bloomin' volcano, like they're doing!