The metal ring on top of the secondary. It does three separate jobs, and on a ramped coil the second one becomes so much more important than the others that the whole department ends up doing the opposite of what everyone else does.
The three jobs
It stores charge. A discharge is charge leaving, and a topload is the reservoir it leaves from. Without one, the resonator has only its own winding capacitance, which is a few picofarads.
It sets a good part of the frequency. That capacitance and the secondary's inductance are the resonator. Change the topload and you have changed the coil's frequency, which means you have changed where the primary has to be tuned.
It holds the breakout and keeps the discharge off the winding. The field on a smooth toroid is spread out, so nothing breaks down until you put a point on it. Without a topload the discharge leaves from the top turns of the secondary and burns the wire down.
The cost, which is the same everywhere
More capacitance means lower impedance:
Z = sqrt(L / C), and volts at a given power go as sqrt(P · Z)Fewer volts, and volts are what make the arc long. So a topload is a trade in every machine: you pay in voltage and you buy stability of frequency.
Whether that is a good trade depends entirely on how much your arc detunes you. A slayer exciter at a few watts has nothing to detune and wants a point and nothing else. Burnett runs a 500 W HFSSTC with no toroid at all.
And why this department buys it anyway
A metre and a half of arc adds capacitance comparable to the topload's own, and the resonance walks down by tens of per cent across a single ramp. The driver has to track that the whole way, and the further it walks the harder the tracking is.
The bigger the resonator's own capacitance, the smaller a fraction of it the arc represents, and the less the frequency moves. So a QCW takes the voltage penalty on purpose, and then goes further: Steve Ward put a series of small capacitors inside his secondary on the early versions, because he needed more self-capacitance to withstand the detuning from long sparks.
That is the opposite instinct from every other coil, and it is the right one here.
The move that is not a move
Doubling the toroid to halve the detuning does not simply work, because the absolute frequency at the end of the ramp falls with it, and the absolute frequency is what keeps arcs straight. Compensating with turns then drops the secondary's impedance, which raises the current and shortens the ontime. Three things move and only one of them is the one you wanted.
That arithmetic, with the numbers, is why a QCW adds capacitance.
Where to go next
The general trade and everything about building one, including why stacked toroids do not add up and why the breakout needs capacitance to the topload, is what a topload does. And what the discharge does once it leaves is what a QCW arc is.
The toroid on the QCW diagram is drawn wider than the other departments', and its secondary is shorter. Both halves of that are this decision.