How was it generated just by a change in abstract information about a mental process?

Abstract information? They were told they were being prayed for (and I suspect as christians) they were expecting this top have an effect - otherwise even as christians they would be expected not to believe in the power of prayer.

The important control group of this experiment were the two others

Group 1 received prayers and didn't know it; Group 2 received no prayers and didn't know it; there was no difference between the two - so prayer did not help.