Havoc always copies spells to the target marked with the debuff, and therefore you always have to switch back to a new target after applying the debuff. For this reason, some players prefer the use of a mouseover macro to apply Havoc. Havoc only copies single target spells, and fortunately it also duplicates the Soul Shard generation of its beneficiaries. This means you can hold onto Conflagrate charges for some really quick Soul Shards while Havoc is active.