To read about the Brood War equivalent, see Extractor Trick.
The Extractor trick allows you to build a unit over your supply cap. This is performed by using a Drone to create an Extractor, which is the cheapest structure. Doing so frees up one supply, so you can build another unit (usually another Drone). Finally, cancel the Extractor to get your drone back. Since an Extractor costs 25 minerals, the cancellation fee is only 6 minerals.
Doing this trick is considered economically inferior to a standard 9 Overlord.
 Build Order
The basic Extractor trick is as follows:
- Drones until 10/10
- 10/10 - Extractor
- 9/10 - Drone
- 10/10 - Cancel Extractor > 11/10
The Extractor trick is most commonly used to make an additional drone just before or after the first Overlord is produced; the exact timings will depend on the specific build. Doing this gives your economy a slight boost in the early stages of the game.
However, the extra unit can be used for other purposes such as scouting, making an Extractor at your opponent's base to slow their gas down, to place a natural expansion, or to use it for certain aggressive builds currently in the game.
 Double/Triple Extractor Trick
Some players have designed builds in which you morph 2/3 drones into Extractors in order to go negative 2/3 in supply, respectively. For example, in a game at MLG Raleigh, CatZ uses the triple Extractor trick in order to get out some fast Zerglings for some unexpected quick pressure.
Whether the economical gain weighs up to the 6 mineral cancellation fee depends on the specific build. Some players use it to keep their fingers busy , but thanks to Skrag , the data has been spaded. The findings are below.
 Skrag's Post
The 9 Overlord is the best economic build, period. Minerals at 2:15: 370
10 Overlord can be used for 10pool and overpool, but is inferior to 9OL in every other possible respect to the tune of 10 minerals, and loses the small window of advantage it gets by building the 10th drone sooner by the time 10OL finishes its 11th worker. Minerals at 2:15: 355
A single Extractor trick doing the trick before building the Overlord (11 drones then Overlord) gives a small time window of about a 15 mineral advantage, between workers 11 and 14, but there is a cost, of around 5-10 minerals. Best case, you *might* be able to break even when compared to 9OL, but most of the time you'll lose 5-10 minerals. Minerals at 2:15: 364
Single Extractor trick after the Overlord (10 drones, Overlord, trick to 11 drones) is inferior to 10-T-OL in the 11-13 worker range, and is inferior to 9OL in the 14-15 range, so I honestly see no reason to ever trick this way, UNLESS you meant to 9OL, but forgot the Overlord. In that case, this is your best way to try to catch back up, because the cost is 0-5 minerals instead of 5-10. Minerals at 2:15: 369
The double Extractor trick provides such a small advantage over the single Extractor trick during even a smaller window that it should probably only be used exactly to 12 pool, and even then, there is an extra cost over the single Extractor trick. It probably compares to the single Extractor trick much like 10OL compares to 9OL, meaning that if you're doing something exactly on drone 12, you might use it, otherwise, the single trick is probably strictly superior. Minerals at 2:15: 353 (imprecise, but again, likely strictly inferior to single trick except for fastest 12pool)
More drones earlier to defend against fast rush as well as improved utilization of the initial Hatchery Larvae.