They do both. The microphone/speakers are default "on," but the circuit is interrupted when a pressure wave (i.e. the loud noise) that meets or exceeds the designed maximum reaches the microphones. Then the circuit is interrupted, and the speakers do not transmit the sudden, loud noise.
As far as bone conduction goes, I don't see how it could really be significant (at least for brief duration sounds). If the pressure wave has enough energy to vibrate your skull strongly enough to damage your hearing, it's going to cause more than a little discomfort while doing so.
And speaking of plugs, I just got my SureFire Sonic Defenders PLUS today. I've been wearing the originals for a while now with excellent results, but sometimes the seal gets broken, especially when I am sweating. The PLUS version has an extra flange on the part that is inserted into the ear canal, so the seal is a lot better. I also got the large size this time (they are sized based on the size of your external ear, not the canal), and they are a lot less prone to twisting (which may have been part of the problem).
NRR is 19 dB, but that is (as far as I can tell) for steady, low intensity sounds. When a suddent high intensity wave hits them, the attenuation is better, and the sound level never gets to where it can damage your hearing. Someday I may cut an old pair open to examine what kind of design they are using, but what matters is that it works.
I also like that I can hear normal sounds better with them, especially with the little plugs not plugged into the holes that lead into the the baffles or whatever they use internally to decelerate the pressure waves. You can shoot okay with them like that, but when I'm shooting something like an AR (or worse), I plug the holes just in case.
You can buy them anywhere that sells SureFire lights (like Gander Mountain) for $10-11 bucks, whether you want the original or PLUS. However, I've only seen the medium size at Gander mountain, so if you want large (or small), order from
SureFire Flashlights & Tactical Lights.
Oh! I forgot to say why I wear plugs instead of my electronic muffs most of the time. When I shoot a long gun, my gargantuan cranium causes even the slimmest earmuffs to bump into the stock, which either prevents me from getting a proper cheekweld/head position, or if I force it, it breaks the seal on the muff. Then, in the summer time, muffs are just too hot even when I could use them for pistolas.