it only blocks the whole effect if its 'stronger' then the imbue (i doubt its possible). if the fire shield is decreacing the imbues effect by 50 points you can still be frozen by an effect60 ice hit (random numbers) but only for a very short time in this case
its useful (i think mainly in pve) cos it allows you to use small pills insted of med ones and med's instead of large ect. it does the same in pvp, but you have to reactivate it once its over, so losing some important seconds.
but used with some effect decreacing jewels it might own
