Well, with the new change to elemental mastery in which it gives one instant spell on your first cast, it does NOT work like PoM. The new elemental mastery lasts 15 seconds, and although it says only the first attack is instant, heres the catch. Since it lasts 15 seconds (for the 15% crit at least) rather than disappear after one spell, if you, say, cast lightning bolt from far away and its still in the air once the GCD is up, you can cast another spell instantly. So step by step
1. Pop elemental mastery
2. Cast lightning bolt or lava burst (not chain lightning since it is an instant travel time)
3. If you are far enough away from the target, it will take more than 1 second to get to him, and therefore you GCD will be up and can cast yet another instant attack.