How do you remove the special hose bib valve that is on the bottom of the hot water tank?
First I should answer the question as to whether it will fall out if it is unscrewed too far. The answer is NO. After 4 turns to open it, it will just spin around without moving further out while the "O" rings should keep it from leaking.
If it is clogged up and not much is coming out after opening it, you could wait until the water was no longer hot, and totally remove the valve -- if 40 to 60 gallons of water on the floor presents no problem for you. (not recommended for third story apartments) If you cannot do that, then first try running water out of this valve while there is still water pressure in the system. This will often force the clog through the valve and then allow you to turn off the water to the heater, open a hot water faucet in the house (to let air in) and now it should flow, even if slowly. (always remember to turn off the energy source to a hot water tank if you are draining it) Once it is empty, you could then remove the valve and clean it out properly. Follow this link for more details on flushing sediment out of a tank.
IF IT JUST SPINS, HOW DO I REMOVE THE VALVE
But just how do you remove that valve? First you turn it left (counter clockwise) to open it at least 4 full turns. This will get past the closing threads and into the free turning portion of the valve. Now pull the valve out, it should slide a little bit and then it engages the retaining threads, which are left handed threads. While pulling it out, turn it to the right (clockwise) at least 6 full turns and now it should pull out.
To put it back in, or replace it with another (old tanks are good sources as these valves rarely wear out) just reverse the steps above. Push it in to the threads, turn to the left while pushing it in until it turns free. Push it in further and then turn it to the right to engage the closing threads and close the valve.
I only know all this because I found an old tag under the tank during a renovation. The tag said: "Don't loose this tag."