The solid portion of the mash settles pretty quickly - so you effectively have a big pile of husks and large grits etc sitting right on top of your element - nice insulating layer to trap the heat. My guess is that there would be a quite significant difference in temps between the region of solids at the bottom and the rest of the tun. Easy to test ... take a temp at the top, stir the mash up and take another temp at the top, If the total mash gets warmer, that heat came from somewhere else in the tun that was quite a bit warmer before the stir.
You need to stir (or agitate somehow) initially - it gets the the majority starch separated from the grains, and allows it to dissolve into the liquid portion. Thats why it takes a while for the mash temps to settle down - the water is working its way into the solids, hydrating and heating up - doesn't all happen instantly. Stir, let it sit for a minute, stir - when the temps stabilise, then things are more or less done.
If you add heat at the bottom - you are going to be gelatinising a bit of extra starch - but not necessarily allowing that starch enough access to the enzymes if you don't stir. The starch being all down the bottom in the solids, but the enzymes being spread out through the liquid. This will happen anyway as the mash progresses - but heat into the grain bed without stirring, gives one of the weaknesses of BIAB more potential to cause trouble. Not really a biggie because it'll still all convert as you ramp to mash-out/sparge temp. But if you dont do the mash out bit.. I can see an issue.
I am - probably in a way that gives people the irritations - a big advocate of stirring in a BiaB mash. If you want, during the actual mash a few times, but certainly stirring constantly during a ramp from sachrification rest to sparge temps. A well stirred BiaB mash, with a nice fine crush - has the potential to be very very efficient in terms of actually converting all (or close to all) the starch in your grist. More so than any other form of homebrew mashing - thats one of the reasons why if its done well, BiaB gets efficiencies comparable to techniques that involve sparging.
Stir well - Stir constantly while you have any heat on - Stir to a mashout/sparge rest.
That will give you great conversion, good efficiency and help to make sure that the increased level of solids that make their way into a BiaB kettle aren't unconverted (haze making) starches.
Or thats what I reckon anyway.
