Seems like everyone has it straight, but just to emphasize for the benefit of those browsing this topic:
Automatic deployment and automatic activation are two separate and distinct 'mechanisms', if you will.
Automatic deployment means the bracket which holds the EPIRB has a hydrostatic release mechanism similar to those commonly found on liferafts. When the boat submerges to a certain point, the mechanism should release the EPIRB from it's bracket. If it's unobstructed and unhindered, it should then float to the surface and ping away. ALL such brackets can also be manually released, though they can be a bit tricky to put the EPIRB back in the bracket after inspecting them on many models.
Automatic activation refers to a particular EPIRB's capability to activate and begin transmitting when it's in water. It's not a hydrostatic thing, but usually accomplished by continuity between two of the case screws. These are fairly sensitive - I've been called on the radio to check my EPIRB, turned around, and sure enough, the light was flashing and it was transmitting away. The switch on the top was still in the secured position, but it had been taking enough of a soaking in the 5 minutes that the boat had been underway for it to activate. 5 minutes for it to energize, ping satellites, go back to the ground station and LUT, from there to USMCC in Suitland, Maryland, from there to CG D-1 in Boston, from there to the appropriate Group (now Sector), from there to the appropriate Station, from there to the boat.
I've also chased these all around the harbor because they were stowed improperly and water sloshed over them. On most, if not all models, once they're activated, they're activated. Can't just shut them off like a radio. You have to disassemble and remove the battery.
Worth noting, though, is that ALL automatically activated EPIRBs ARE capable of being manually activated, and all current models will give some visual indication that they are transmitting. Many also have a strobe which, while it isn't the greatest strobe in the world, is better than nothing. I can't overemphasize the utility of a "high intensity white light flashing at regular intervals from 50-70 times per minute". Persons in the water are very hard to find at night. High intensity strobes in the water are hard to ignore at night.
So, the modern 406MHz EPIRB itself. Four features that help you get found:
-Composite signal on 406.025MHz. This signal broadcasts a hex number to satellite(s) in the COSPAS-SARSAT constellation, and to equipment piggybacked on other LEOS/GEOS satellites. Satellites send it to Earth, computers figure out positioning data, SAR Controller looks at registration info (REGISTER YOUR EPIRB! It needs to be shouted again - REGISTER YOUR EPIRB!), can't verify vsl is safely in port, launches appropriate resources to the last known position, or to a commence search point.
-121.5MHz signal. That's right! 121.5 didn't really go away. What went away was the EPIRB's that only transmitted on 121.5. Modern 406's primarily transmit their composite signal on 406.025, but also transmit a low powered signal on 121.5 to allow for final radio direction finding. Really, really cool feature.
-GPS enabled, akca GPIRBs. These include positioning data in their own 406MHz transmission. This provides for two sources of positioning data, or in the case of some EPIRB transmissions that can't be immediately positioned, faster positioning data.
-Strobe lights. Not the brightest and best, but a lot of EPIRBS do have a strobe light. Better than nothing.
So, the EPIRB tells you that you need to be searched for, who you are, and who might be able to rule out/rule in that you're actually underway, and where you are. The boat has a DF set to 121.5 and looks for the signal en route to the position from the EPIRB (either resolved by the satellite and computer magic, or by the position encoded in the transmission). Boat picks up that signal and steers a few degrees off of it (you don't want to steer directly at potential persons in the water - it could do great harm to them). Boat crew sees strobe lighting up half the sky (because someone has real strobe lights in addition to the one on the EPIRB). People get picked up. People dry off and go home. Hours upon hours of paperwork ensue for all parties involved.
