ORIGINAL: blw
MJD,
I would agree that the analogy of the tires running out of grip would work. However, the engines react differently when getting enough grip as opposed to getting enough air. Model engines speed up as they unload. How much do they speed up? There is no hard and fast rule. Most people I've heard from give a headroom of about 1000 RPM.
In the scenario suggested by FF, I believe he is saying the engine "slips" on the ground, so the engine is artifically unloaded to a degree and rpm is higher. Once launched, as the airspeed increases and the angle of attack of the relative wind on the prop blades becomes less, the prop would then bite as you say, the load will increase suddenly, and the engine may load down again. With a slippery airframe he says the airspeed may increase further to allow it to unload again. In a regular scenario, the engine launches slower, then unloads in flight as the angle of attack decreases on the prop.
Now, I thought the drag was higher with stalled blades, but perhaps this is not stalling but what, cavitation? I dunno, I'm no fluid dynamics expert.
MJD