Well I got the robe this morning. The buff lasts 30 seconds. With extended duration aa for me that was 42 seconds. However it drops after 2 casts, whatever the spells may be. It is definitely NOT a straight 40% final resist drop. I used it today raiding Warlord Gintolaken in PoEB ring, the final boss off the first ring after zoning into earth. We fought him twice, first as someone else's leftovers then as our own ring. He is nigh total MR but *not* flagged immune to slow etc.
Without the robe's effects he resisted amongst total raidees about 98 in 100 slow attempts using the low resist slows, 30 from enchs and 60 from shamen. Two golden beebees in all of that landed. With the robe applied he also resisted 10 of 10, neg 30 mr attempts from me. Were this a straight 40% if-it-was-going-to-resist-it-lands-anyway I would have expected to see four land plus or minus some. None landed.
I'm sure this is designed to reduces resists in some fashion but I doubt its a flat 40% of Ha!-didnt-resist-after-all.
Meredeth, Amicus, Tunare