First, you can only put focus on staves (spell crafting). What focus does is basically increase the stats of the given focus. If you have a focus ability, you can't do anything else while that ability is in use or it will break the focus. In other words, if you have earth focus and you cast an ice spell, you'll break the earth focus and the stats for earth return to normal. So, if you cast say a lava DOT and then cast an ice spell, the bonus for earth won't apply to the DOT because the focus was broken by the ice spell. Hope that helps.