A compression ignition engine will, um, ignite the charge at some point of the compression stroke. With spark ignition, you control this by setting the point where the spark goes off. With glow engines, you have to do it some other way. It's a combination of compression ratio, fuel (mainly, how much nitro is in it), prop, and glowplug (and probably one or two other things I've forgotten). All other things being equal, a "hot" glowplug will set off the charge at lower compression than does a "cold" one, effectively "advancing the spark". When the charge goes off too early, it's the same as setting the spark timing too early - you get pre-ignition, or "knock".

Iskandar