@Intruder, technically it's still an "thrust limit exceedance", not an overboost. Tdracer's argument that it "might" exceed limit in the compressor is something that we can't know. Probably it is wrong too, I read somewhere that the CFM56 was run at 140+% during testing. Probably there is a much wider margin for the compressor than what we are able to exceed with the engine on the wing.
Cosmo, don't confuse absolute limits with 'certified limits'. Redline is a certified limit. However the engine manufacture has to demonstrate that the engine won't structurally fail until (IIRC) at least 25% over redline. That's what GE was doing when the ran the CFM to 140%. Similar limits apply to pressures. Turbine engines seldom 'blow up' - in no small part because these absolute limits are so much higher than what the engine should ever see. But they do occasionally blow up, and engine failures of any type become significantly more likely if the "thrust limits are exceeded". There was a 747 freighter crash not too long ago where an engine failed at B1 - the crew responded by advancing the throttles, overboosting - er - exceeding thrust limits on the remaining three - at which time a second engine promptly failed. They were unable to make it back to the airport and crashed. It doesn't matter if you call it "overboost" or 'exceeding thrust limits' - it's not something a pilot should do.
JammedStab - the PW4000 uses an electronic overspeed (which effectively shuts the engine down when tripped), CF6-80C2 has a hydromechanical overspeed - basically a "fly ball governor". The EEC/FADEC will still protect rotor speed redlines, but if something breaks such that the engine exceeds N2 redline by ~1%, the overspeed governor will start bypassing fuel back to the fuel pump to maintain N2 at ~ redline+1%. It won't shut it down. The RB211-524 on the 747-400 works similarly to GE, except of course using N3 instead of N2.
I was involved when we first went to FADEC, and there were a lot of people that didn't think we should use electronic overspeed protections - if we couldn't trust the electronics in the EEC to protect the engine, how could we trust the electronics in the overspeed protection to protect it
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However as we gained experience with FADEC, must of that resistance has faded. GE was the last holdout for mechanical overspeed protection - using it on the CFM56-7 (737NG) and the GE90. But the GEnx has electronic overspeed protection.