Dragons and those other elite races had 'indestructible' circuits, which was why Darius could keep practicing without harm. The reason he felt a 'breakthrough' when he succeeded was because he had forcefully strengthened his mana circuit until it could handle Master-grade spells.
Using the car example, if one created an 'indestructible' car with internal parts that could never explode and forced jet fuel into it, either it would refuse to start or eventually forcefully adapt.
Of course, it was not that simple, but it was the best possible analogy Darius could think of.
The Itemancer breathed out and understood what he had to do. When training his army or elite forces of any kind, he would now focus on magical circuit strengthening and body strengthening.
The same theory had to work for those who Sparked. They could not use abilities above their stage because their bodies had not been strengthened by their base ability to withstand such a thing.
