Its a way of getting homogenous heating throughout the grain bed.
When u heat the water at the top above the grains, the digital thermometer reading is always low even though the temperature of the water is much hotter than this to touch, and is physically boiling. It's because the grains underneath it are alot cooler.
After you transfer it to the grain bag though and continue heating underneath it, everything is then mixed up real swell and it works very nicely.
This comes from the fact if u just heat from underneath it from cold, the thermometer is much less stable. First it jumps up from like 58C to 62C, then can overshoot all the way up to like 70C. Then you stir it around and its back to 58C again in no time. Thats because of heterogenity.
By my method though, the whole pudding is kept very HOMOGENOUS, and thus is just an interesting spin on things that I have been personally developing.