First of all, this strategy usually doesn't work that well against terrans since they block out their entrance. But against other races, they're about as good as dead.
The requirements to make this assault work: 10 or so overlords, about 20 zerglings, a humongous force of units that you'd like to kill your enemy with (ground)

1. Research speed and transport for overlords.
2. Load the 20 lings in separate overlords.
3. Send your overlords over to your opponents base (mineral area) to drop the 20 zerglings, they won't last that long.
4. While he's thinking: AHHHH!!!! HE'S DROPPING!!!, he'll send all the available units he has guarding his entrance to defend against the drop.
5. While he's busy trying to organize his forces to defend, you can just send in the frontal assault to kick his @$$.

CreepColony's Opinion: Of course this will only work if you can get your overlords into their base.