As currently Rune line of spells, along with Berserker Str family line of give you an amount of damage to absorb before wearing off.
Right now, if you use these spells and are hit. You gain the amount of the hit the NPC/PC hit you for.
As example Enchanter or his Target is at 80% of life. They are buffed with Rune line of spell
Increase Absorb Damage by 245 (L40) to 394 (L50)
Said PC is hit for 25,50,30 ect up to the limit of the rune spell, are healed for the amount of each hit up to the limit of the Rune. This isn't classic and bugged atm.
I'm not sure if the wizard/necro line of Skin spells act the same way.