i made an umbrella for my off hand with increased magic power, i use a sickle for my main hand, when i enter the casting animation i lose all stats from my umbrella. once the animation is over i get the stats back, but the animation takes long enough that with certain spells i still have reduced stats, when i cast swarm the spell takes long enough to reach the target that i get the stat increase back, but faster spells i lose out on all the stats in my offhand
