How Does Channeled Spells Work?

[HMS Invincible]

How do spells like Channeled lifetheft work? It's in Complete Mage, and the wording confused me. I know I prepare it normally. But, what happens when I cast it for a full round, and then decide to add another round to it? Does it replace the old damage with the new damage? What happens as I extend the spell through swift, standard, fullround?

[Starbuck_II]

How do spells like Channeled lifetheft work? It's in Complete Mage, and the wording confused me. I know I prepare it normally. But, what happens when I cast it for a full round, and then decide to add another round to it? Does it replace the old damage with the new damage? What happens as I extend the spell through swift, standard, fullround?

You cast it by decide how much you want to expend as an action.
So you choose Swift, move, full rd, etc what it says.

Main benefit is when you can't afford a full rd, you have option for lesser effect by using less action.

[HMS Invincible]

Ok that makes sense, but it also says that you can channel for 2 rounds. Doesn't that mean nothing will happen in the first round?

[The Random NPC]

Correct, nothing will happen for the first round, but the second round something even greater will happen.

[HMS Invincible]

I would say that the spell has only 2 important parts, the quicken fatigued +5 temp hp, and then the full round action part which exhausts, hurts 5d4, and I gain temp hp. The 2nd full round action is nice, but I can't risk using 2 rounds, which I'm vulnerable, to disable/nuke one enemy.
I would pair this with an accelerated chrono charmed summon monster III. I can take advantage of the enemy's weakened dex and str. Or I can fire off an orb of fire and the swift version of lifetheft. Either way, the debuff is roughly -1 to dmg, ac, and attack. I wouldn't have went out of my way to learn, but I guess it's good enough just because of how versatile the swift-full round option is.
How would you judge the quality of this spell?

[TurtleKing]

That is just one of the Channeled spells. You also have Pyroblast that is basically the D&D version of Dragon Slave. Another that channeled for 2 full rounds act as a Cure Critical at long range. There is another Sonic Blast that if channeled for max lets you nova. A Bard can get that spell around his 13 level. Considering available to Bards if you can enhance the spell with Amplify then could possible affect a large area dealing alot of damage.

[The Random NPC]

I'm unsure, but I remember being able to declare that you were going to channel the spell for X time and then cut it short if you needed to.