I tend to think of spells as being less like a button or a lever, and more like a high striker. If the spell is the ringing of the bell, then you've got to put sufficient energy in to attain that. More energy will allow you to hit the bell harder, and thus ring it louder, but you have to be able to put enough in to reach the basic threshold in order to ring it at all.

Of course, spells routinely output more energy than they could be getting out of the metabolisms of their casters, so for the analogy to hold up under extension, it's more like an electronic high striker, which sounds a siren at different volumes depending on how hard you hit the target.