It is the speed at which the temperature shoots up, not the temperature itself. My dn has them when she's ill and never really has a high temperature, certainly never above 40 but does fit.
They found that sometimes meds can exacerbate the problem because they would give them regularly until she fell asleep and they would wear off. The artificial lowering of the temp meant that as soon as the meds stopped working, her temp rose quite quickly and this seems to be the key time for the fits in her case.
They have had much more success with keeping her cool by natural means and her temperature constant with very loose to little clothing, no blankets, fans, cool drinks very regularly. No cold measures though, like cold baths etc as this causes the same problem, the body fights to raise the temperature again when they start shivering.
It's difficult because the temperature is part of the body's natural defence against illness and the body will fight to raise the temperature as the meds wear off.
It's so frightening. I hate seeing dn fit, but I always try to remember that it's a very clever defence mechanism and not actually harmful. And she will grow out of it thankfully.