We ended up need to make a rules clarification on Flame Jet - and by extension, all 1-second duration spells - in my GURPS Dungeon Fantasy game.
In pre-4e GURPS, Flame Jet was clear:
Turn 1: Concentrate.
Just before Turn 2: Roll for Flame Jet, pay for the spell.
Turn 2: Attack with the Jet. If you don't still need it, simply don't maintain it.
In 4e, you get an issue:
Turn 1: Concentrate and cast Flame Jet. Pay for the spell.
Turn 2: Maintain Flame Jet. Use it to attack.
Turn 2: Use it to attack. Maintain or don't maintain Flame Jet.
For example, in 3e a Flame Jet-15 caster with a 4d Flame Jet pays 3 points to use it once. A 4e caster pays 6 points to create it and use it once. Or 3 points to create and use it one, if you maintain after. So when you maintain is a big deal.
It really depends on when you maintain spells. GURPS Magic doesn't seem clear on this, nor is GURPS Basic. Mostly, it doesn't matter - spells tend to last long enough that a second either way doesn't affect things.
So we ruled that it's "maintain at the end of your turn." Do you stuff, and then pay before the next guy goes. It just happens automatically when needed, and deciding not the maintain is (of course) a free action. This approach seems to cause the least damage, and gives you a full turn of utility for those 1-second or otherwise very short duration utility and movement spells (Walk Through Earth for example).
But it's interesting how the shift from "end of your turn, which is just before you decide on your next action" casting to a cast-and-it's-there approach changed things. No 0-second casting time spells saved a lot of headaches, and this makes the whole process more clear, but it doesn't seem like when you maintain is ever formally addressed/ Or maybe it is and I just can't lay a finger on it. A subtle change that didn't really come up until last game, which shows you how rare it is to use short-duration spells in combat in my games.