If you release early, the spell won't go off. For max RoF, watch your Magicka Gauge, It stops going down when it's ready to fly. This goes with all spells, not just dual cast. Eventually, you'll get the timing, and should be able to stun-luck 2-3 mobs at once. Also practice half stacking, to fire one as the other charges for rapind fire. This doesn't stagger, but throws a lot more damage, faster, and for less Magicka. (Unless you're running the 100% free Destruction Enchantments. Even then, it's still faster.) Dual Casting doesn't affect the damage from Runes, Cloaks, or Wall spells, nor do they stagger. Also, Shockwall seems to interrupt Impact, so it doesn't work at all. Impact is just 1 of the many tactics availible. It's effective, but Magicka inefficient, and lowers your damage dealing. Learn all of them. Mages are supposed to be Intelligent, not dumbed down to a 1 trick pony. If you know what to use, when, it's more powerful than anything other than a maxed out Warcrafter.