Its a very weird item to wrap my head around as well. From my understand, I only have to reroll successful rolls that would of passed my base save ( in this case, a 4+), The benefit comes from that a 2 or 3 would fail my base, so I dont have to reroll them, but then when you add the +2, They pass! This is of course against no rend. Once you add rend, its a little trickier. since at -1 Rend, a 2 is still a fail, or -2 Rend, a 3 is still a fail! The Rerolling 1s from the Sorc Lord Buff just helps keeping my chances of flat out failing on an initial roll of 1 to a minimum.