The idea behind the early second hatchery is to get more resources than your opponent. This is easily accomplished on maps as Lost Temple, Rivalry, Ashrigo, Bone Canyon, Winter Conquest, etc...where there is a close expansion to your main base, and which your opponent must pass through first.
Now most races cannot stop this, since sunkens provide such a great early defense. However this usually leads to containing yourself, since you have no mobile units. You shouldn't try to get too many zerglings, since you really should try to out resource your opponent at the beginning. Put a few sunkens up to hold off intial attacks, probably around 3. Make sure to scout your opponent and make sure he doesn't decide to bunker or cannon you in. However keep in mind that sunken colonies have a longer range than a bunker.
Basically you can power up heavily, since you will have enough resources to support 4 hatcheries. This is really the only way to use this build order effectively, since you will get more resources than you have larvae. Now to start this build order, you will want a spawning pool first, since getting zerglings is more important early on.
Now you should send a drone to your expansion and time it just right so that when you arrive you will have 300 minerals. Make sure you have zerglings now to kill off any scouting units. As soon as the second hatchery completes, begin making drones at both bases to create sunken colonies. Now once you are satisfied, make more drones to get the resources needed to get hydralisks or mutalisks. You should also get a few more hatcheries as soon as possible.
On other maps, where you don't have another expansion, you can still build this way, except you may want to build the second hatchery first, then a spawning pool. This is because the spawning pool can be created fairly quickly and once completed you have to creep ready to place sunkens down.
