Phil just dumped eight out of sixteen shifts into the spell, so there are eight shifts left, which you can supply in the following way:
1) Decide how much power you want to supply in this exchange. For each shift you go over your Conviction, you take a hit of mental stress, so you could channel up to three shifts without taking any mental stress whatsoever.
2) Roll Discipline to control those shifts of power. If you succeed, you channel those shifts into the spell. Go back to 1 until you're finished casting. If you fail, go on to 3.
3) If you fail, every shift channeled into the spell so far comes back as either fallout or backlash. So if you try to finish it quickly and muck up a Discipline roll, you're looking at a nine shift hit at least. You choose how to split it up among backlash (4) or fallout (5), but if any of it is released as fallout into the environment, the spell fails.
4) If you take backlash, you take a mental stress hit equal to that many shifts, which can be pretty nasty.
5) If you release it as fallout, I get to figure out what happens based on the spell being cast and how many shifts of fallout are released. I am also encouraged by the book to be nasty as possible when doing so.
So essentially, you can play it safe and channel a shift or two at a time, or you can risk blowing the spell back on yourself/everyone with a botched Discipline roll and finish in a shorter amount of time. The choice is yours.
