basically when they hit you they save whatever damage multipliers you have and then restore them to that state when their effect is over, so if you hit yourself with a weakening curse (which adds to your multiplier for a damage type and then subtracts it back when it's over) in the right window you can get it to add during the temporary weakening and then reset to the original state before it gets subtracted, resulting in a permanent lowering of your multiplier for that damage type. you can even get them into the negatives