Sustained Combat Spells?, Doable? If so...how do they work? Options

[Cardul]

OK, I was looking through Street Magic, and I noticed something:
By the rules as written, unless I missed something, I can make a sustained combat spell.
This raised an interesting thought:
If I make such a spell: I will assume I have to maintain LOS the whole time I am sustaining it, but...
Do I have to make a roll each combat pass/turn to keep damaging my target? Or does it just go by my initial spellcasting roll?
Would the damage be per initiative pass I make? Or per Turn?
If I were using the optional +1 drain for net successes applied to damamge on direct combat spells, would I have to resist that extra drain each time I do damage?
Did I just make someone's head explode by even suggesting such a thing as a sustained combat spell?