The effect of Mana Wall appears to be 100% damage transfer to MP.
Reduces damage taken by 50%. (100 Damage taken takes 50 mp away instead of 100 hp)
All forms of damage modifiers (PDT, Phalanx, Stoneskin, etc.) are taken into account before mana wall effect's takes away mp.
Because no HP damage is taken, this ability effectively prevents spell interruption from damage.
If you get reduced to 0 mp because of damage taken from Mana Wall, Mana Wall wears off immediately.
Damage will carry over to HP when MP is low enough.
Take note, that being hit by enemies while resting with Mana Wall up will cause you to stand, even with Stoneskin effect up.
Mana Wall does work for DOTs, but having a DOT on will still wake you from Sleep and interrupt resting, even though you are not losing HP.
While Mana Wall is active Sublimation's effect changes from -2 HP per tic to -1 MP per tic. When your Sublimation charge is used, you will still regain 2 MP per tic that it was active. Additionally, the maximum duration for Sublimation charging is still limited by your maximum HP.