I think the electronic rapidstart ballasts that are common in the US, aren't those low cost electronic ballasts that preheats the electrodes by means of a PTC within 1 secs and less, but actually no more then electronic versions of the HPF rapidstart magnetic autotransformers (The ignition principle and lamp starting behavior are the same on both the electronic rapidstarts and the HPF magnetic rapidstarts [Lamp start dim for several seconds and flickers, then come to full brightness, similar in some way to the UK semi-resonant start ballasts]).
This means that in Europe, electronic rapidstart ballasts are really nonexist, and all CFLs that i saw that preheats the electrodes and strike the lamps not by a programmed means, are probably just a simple, low cost electronic preheaters that works by a means of a PTC.
This what i thinking.

All HF electronic ballasts use in fact "resonant start": The resonance effect (series LC) is used to boost the voltage for starting.

How they differ is the way, how they handle heaters and how (and if) they control the voltage during the starting process.

- Instant start use "brute force" - high enough voltage to ignite even cold lamp. In these the voltage does not have to be accurately controlled, only some means of ballast protection is necessary for the case of total lamp failure (e.g. no vacuum). Typically lamp filaments connected in series with the resonance circuit components do the job (they blow up by the excessive current associated in the circuit - the most frequent configuration of cheap 230V ballasts), sometimes a means of voltage (and so current) control by adjusting the frequency (US instant start with two leads per lamp)

- Rapid start control the voltage so, the lamp ignite only when the lamp electrodes are hot enough. As electrodes gradually attain emission, the discharge start to build gradually as well, sometimes with instabilities (the flicker - ignited arc clamp the voltage, so reduce the filament current, so it cool down, so the discharge cease and so on).
The schematic topology may be (and frequently is - you cannot distinguish without deep analysis and calculation) exactly the same as the 230V style instant start (selfoscillating inverter, ballasting inductor and resonance capacitor in series with filaments), only component parameters are chosen (and controlled) so, the voltage is within quite tight tolerance for the ballast to ignite in the RS mode.
Issue with these is the great sensitivity of the voltage on many of used components, so repairs are quite difficult (as e.g. on blown out resistors you can not read, what is the nominal value).

- Reaching reasonable cycle life as the RS concept offer, but do not make the ballast so sensitive for components is usually done by the programmed start concept: First only heat up electrodes, so keep the voltage safely below the ignition level (so e.g. by a PTC short) and in second phase let the voltage rise virtually without any limit (as in the IS ballast) to ignite the lamp. The control may be simple PTC shorting the resonant capacitor (so allowing the ballast current to flow only trough filaments and switch off itself after some time) or more complex semi-resonant frequency control (working above the resonance, so currents are high enough to heat up electrodes, but voltage still low enough to not ignite).
With separated timer they start sweep with the frequency across the resonance after defined preheat time. This frequency controlled preheat is implemented as the part of the ballast driver IC's, as it does not require any extra power component. The consequence is, then such ballast is limited to given lamp type and cannot control other type, even with same current (as it may ignite prematurely).