It looks like a european patent application for Rossi's Fluid Heater patent was recently published. The Documents tab in that page contains a link to a provisional US patent application describing what the "Rossi effect" is.
https://patentscope.wipo.int/s…621/PDOC/WO2016018851.pdf
Relevant excerpt:
In the european patent application there are a few new paragraphs compared to the USPTO version which refer to this provisional patent application.
QuoteIn one embodiment, the reagents are placed in the reaction chamber at a pressure of 3-6 bar and a temperature of from 400 C to 600 C. An anode is placed at one side of the reactor and a cathode is placed at the other side of the reactor. This accelerates electrons between them to an extent sufficient to have very high energy, in excess of 100 KeV. Regulation of the electron energy can be carried out by regulating the electric field between the cathode and the anode .
QuoteIt should be noted that other heating sources can be used, including heat sources that rely on combustion of, for example, natural gas, as well as heat sources that rely on electrical induction. The use of gas thus avoids the need to have a source of electrical energy for initiating the reaction.