Basics of Starcraft 2 Drops
Starcraft 2 drops is a phrase that refers to using either a dropship, an overlord or a warp prism to drop units behind enemy lines, skirting your opponents main army and striking at his mineral line, tech buildings or production facilities.This is especially useful if your opponent is beating you in production, has a larger army or has fought you to a standstill. It is also effective on large maps with many expansions if your opponent does not have the mobility to defend all of his bases. They can also be an effective way of breaking a contain, whether it’s Photon Cannons or bunkers. In addition to simply dropping units in a base, there are many units that are able to sneak behind enemy lines and attack vulnerable spots without being detected or with the ability to escape easily: intelligent use of these units can be the key to winning a losing battle.
Terran Drops: Using the Dropship, Reapers and More
Terran players have some of the most effective drop tactics in the game: because their transport units also heal, they are able to do more than just harass the back of the enemy base: a M&M&M attack can strike powerfully at the entire enemy force. Stimmed Marines and Marauders can also destroy enemy Nexuses and Hatcheries quickly and easily if your enemy doesn’t respond. Whether you just want to drop a handful of troops and destroy workers or execute an entire attack, dropships are an excellent way to provide mobility and tactical superiority to the Terran army.
In addition to dropping Marines or Marauders, it can be eseful to drop Thors, which take down workers extremely quickly and are difficult to kill. Reapers are also very handy for destroying bases, and though they are rarely seen outside of the opening minutes of a game, they can be used to devastate tech buildings and worker lines if you have APM to manage them.
Protoss Drops: Using the Warp Prism, Colossi and More
Though the Warp Prism is slower and cannot hold very many of the powerful Protoss troops needed to make an effective drop, it comes with an extremely useful mechanic that makes up for these lackings: it can deploy into a mobile pylon, allowing the Protoss player to warp in reinforcements or even build defensive cannons beneath it. Common strategies include dropping immortals behind the enemy workers - their powerful attack will devastate weak gatherers and can quickly destroy supply buildings and exposed tech. Dropping Zealots with charge or Dark Templar on an unsuspecting enemy can be very effective, especially if you coordinate with a frontal attack, or hide the Warp Prism and use it to warp in reinforcements.
Colossi can also be useful for harassment - their cliff walk ability means they can walk up and destroy entire worker lines with their line attack, but be careful - Colossi are expensive and vulnerable to air.
Zerg Drops: Using the Overlord, Nydus Worms and Roaches
Zerg are both lucky and somewhat limited by the fact that their transport unit also provides their supply. The required Overlord upgrades needed to make them useful as transports are expensive, but since they are very cheap, can often pay off. Since Zerglings and Roaches are cheap, it is very cost effective to send Overlords behind enemy lines to harass workers with Zerglings and try to pick off buildings and expansions. Dropping ultralisks in an enemy base when the enemy army is away can be a game ending maneuver.
Nydus worms are also expensive, but extremely effective at turning a battle around if you are surrounded or contained - if the worm comes through alive, the Zerg player can continue to feed troops into the enemy base. This also works well if timed with a frontal assault. Consider also that Roaches and Infestors can move while burrowed, making them excellent units for attack mineral lines and exposed expansions.