Mason's awesome Seraphim build introduced a mechanic that allows concentration spells to be cast indefinitely by manipulating the atronach stone with fortify restoration potions. This has fascinated me and I thought I'd do a little experimentation with it (and finally contribute something around here!). I need to start by saying that unfortunately I haven't completely cracked what is going on but I may be able to start a conversation that will get us a bit closer. Also there is a tl;dr at the bottom ...
The thing that set me off with this was the simple thought: why the atronach stone? Alongside the spell absorption it grants +50 magicka and as we know both components are modified by the fortify restoration effect. So I wanted to see what would happen if you used the same fortify restoration potion stack but used an item that granted +50 magicka instead of the atronach stone.
And it worked! Drinking a philter, then a draught and then a philter again and putting on any item that grants +50 magicka on a character that has only the 100 base magicka enables the unlimited concentration casting effect. I have tested this with the adept hood, a necklace of eminent magicka and nahkriin. I also tested the necromancers amulet but that didn't work - I suspect it is not affected by fortify restoration. This is tested on the PS3 by the way so if someone could test the same on the 360 and PC that would be great.
So that means that the unlimited effect has nothing to do with spell absorption and is purely to do with the amount of magicka your character ends up with. The philter, draught, philter stack gives a modifier of (((0.4*0.3)+0.3)*0.4)+0.4 = 0.568 - each potion is modified by the fortify restoration effect in place at the time of drinking. 0.568*50 = 28.4 giving the total that we see of 100+50+28.4 = 178 magicka as the game rounds decimals on the display.
I was very suspicious of that rounded off 0.4. I wanted to see if it was possible to get a different magicka value that had a 0.4 with what I had available on my test character and it turned out I could. Drinking a potion then a draught gives a modifier of (0.2*0.3)+0.3 = 0.36. Using an item of major magicka this time gives (0.36*40)+40 = 54.4. And doing this also allows the unlimited concentration casting! Other decimal values do not seem to work but I have by no means tested all combinations ...
It can't be the case that the effect is purely a result of having a magicka value that ends in 0.4. We know that the effect does not work if your base magicka before putting the item on is not 100 so in some way that is changing the equation. This is unfortunately as far as I have got and I am little limited in what I can test on the PS3. If anyone has any further ideas or finds any other combinations of potions and fortify magicka items that produce the effect please add below.
tl;dr: Unlimited concentration casting doesn't need the atronach stone and can be enabled with at least one other combination of fortify restoration effects and fortify magicka values.
Edit - List of currently known combinations of potions and fortify magicka apparel values that produce the effect (drink potions in the order given):
Philter, Draught, Philter with +50
Philter, Draught, Philter with +50 and +50
Potion, Draught with +40
Philter, Draught with +70
Philter, Draught with +50 and +20
Philter, Draught with +20
Potion, Draught with +20 and +20 - Achievable at lv10!
I'm getting this again without any mods installed, can anyone confirm if they can use the glitch successfully on PC version? Setup: Alchemy 20 (start as Breton) w/ no enchant boosts, make a potion of Fortify Restoration and then grab the first Alchemist perk and make a second. You should get 18% and 21%. Drink the 18%, then the 21%, then equip an item with Fortify Magicka and/or Stamina 30 (Novice Hood, any of the Gauldur amulet items). I know I did this with the Novice Hood on my 360...
Store bought potions work fine, it's just something with the rounding of values in potion creation I think. And it's not possible for a mod to alter a core file when installing with Mod Organizer.