As a GM, you might require that the spell's control is always 1 or more points higher than the spell's power. Beating the control check like that means you control the energy especially well, justifying the fine-tuning of the results. (that's actually in the rules; exceptional success means better results). If the control check barely matches the power, you might tell your players that there is a danger a nonlethal spell kills the target. And if the control is lower and they take backlash or fallout? Sorry, you can barely cast the spell without killing yourself; there's no way you can guarantee other people don't die.
Examples of controlled power:
My outsider scion character has a spirit attack that leeches life energy. It doesn't do physical damage in the way of wounds but it does do cellular damage like weeks of fatigue and starvation. It might leave a character taken out with fatigued (mild), starving (moderate), emaciated (severe), skeletal (extreme) and throw them into a comma but it isn't going to kill them outright. It is weapon 8 power-wise; consider how much energy a month's worth of food equals to. It's 15 kilos of fat, protein and sugars. If that much energy was violently released, it would blast apart a small house. The spell just drains a similar amount of energy magically from the target.
Similarly, a weapon 6 area heat attack would heat up an entire area by several degrees (or cool it down by several degrees), including any people inside it. The bodies would collapse from heatstroke without burns. Heating up the entire body by 5 degrees is equal to heating the skin and outer fat layer by 500 degrees energy-wise. That's how you turn a lethal burn into an instantaneous nonlethal take-out; spreading the energy.
