Pain beams function on the principle of constructive interference of waves. Where two separate electromagnetic microwave beams cross they will exhibit a typical interference pattern. At some points the beams combine at their peaks as do the harmonics. These harmonics are at a frequency which is tuned to the optimum frequency of typical water molecules under ambient conditions. Long story short, by focusing two beams at a particular location the water molecules in the targets skin very quickly reach the boiling point. The small amount of water in the skin isn't enough to do ANY harm to the target. It merely briefly boils the moisture in the skin and makes the target feel like they've suddenly been dipped in boiling water. Very painful but not harmful. No one who it's been tested on has been able to remain inside the interference pattern. Most run and scream as fast as they can get anywhere but there. The pain is quite severe but harmless and brief.