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By Shannon “BeefMaster” Rampe

aLive is controlling the map with his hellions, denying Scarlett any quick third base. Scarlett is feeling the pressure but she’s not going to overreact and make too many units; that might cut into her economic edge, and she knows that, against a player like Alive, the economic edge is the only way she’s going to win. Suddenly, aLive’s forces appear between Scarlet’s second and third bases, a large squad of marines and hellions with the healing support of medivacs. Scarlett sends a handful of banelings to slow them down, but the marines snipe the incoming explosive bugs before they can detonate and deal any damage. Scarlett’s only static defense, a lone spine crawler, is in the middle of repositioning from one base to another and is useless. aLive’s forces close on Scarlett’s undefended third base, the drones and hatchery dreadfully exposed. It looks grim for the Canadian hero.

But Scarlett’s not out yet. She quickly grabs her drones and pulls them away from her mineral line, narrowly avoiding a hellion barbecue. The marines are focused on the hatchery, which buys Scarlett only about two seconds. But back in her base, she’s already morphed another host of the deadly banelings and has a squad of zerglings to soak damage this time. They’re moving across the map. A carefully handled split sends the banelings and zerglings in three different directions to flank aLive’s forces. Alive has glanced back at his base for a moment to deal with a momentary supply block and Scarlett crashes her banelings into the hellions just moments before her drones are cooked. The Korean Terran sees his force crushed. He loads his remaining marines into the medivacs and soars to safety. Scarlett’s drones have survived, along with the Hatchery. Alive escaped, but Scarlett bought herself time and managed to keep hold of her economic lead. And she’s already working on a Spire…

This is a description of 30 tense seconds in the middle of a StarCraft 2 match between two professional gamers, Sasha “Scarlett” Hostyn, and Han “aLive” Lee Sook. Players like Scarlett and aLive duke it out at the highest levels of the game for prize money, fame, and glory. Behind them, countless legions of casual players hone their skills in what is widely considered the most difficult and challenging strategy game of all time.

Perhaps you have a friend or a loved one who enjoys StarCraft 2. Perhaps you have heard them talk about “blink all-ins” or “insane marine splits.” (This is not some new calisthenics program in the military.) Wouldn’t it be great if you had some inkling of what they are talking about or why anyone would even care? Well, read on to discover what your friend or loved one finds so endlessly fascinating. Perhaps you may even be tempted to try out the game or check out a professional match at an upcoming tournament.

Believe it or not, hundreds of thousands of people around the world play video games competitively – titles such as StarCraft 2, Dota 2, League of Legends, Call of Duty, Street Fighter, World of Tanks, and others. And hundreds of thousands tune-in to watch online. They’re called eSports, and whether or not you agree that they’re actually “sports,” the broadcasted matches can be incredibly entertaining to watch.

StarCraft 2 is a real-time strategy game, typically played between two players, each of whom select one of three science fiction-themed factions to play: the rough-and-tumble human Terrans, the psychic techno-alien Protoss, or the swarming insectile Zerg. Matches typically last between 5 and 20 minutes. It’s real-time in that, unlike a game like chess, there are no turns. Players play against one another at the same time, without pause. That means that the ability to do things quickly and the ability to multi-task are critical skills, as well as the ability to make split-second decisions. Players must manage complex economies to develop the armies they will use to defeat their opponents, while at the same time controlling their armies on the battlefield, sometimes managing a hundred or more units at once. Players’ split-second tactical decision-making, their strategies, the way they bluff one another or try to conceal their own strategies, and the extraordinary level of control that they exhibit is what makes these games fascinating to watch.

In StarCraft 2, victory is achieved by destroying all of your opponent’s buildings or, more commonly, by forcing your opponent to concede. (This usually happens because the losing player realizes he or she has been forced into a position that he or she has absolutely no chance of winning–the StarCraft equivalent of a checkmate.) At the beginning of the game, each player starts with a handful of workers and a base. Nearby the base are minerals and gas used to build new buildings and expand to more bases, as well as to build armies. Players must choose between focusing early on relentless attacking, building up a strong economy to overwhelm their opponent in the later stages of the game, or managing some hybrid approach of aggression and economy. Players must also balance building a large army or a strong, smaller army by deciding whether and how much to invest in early upgrades to improve their existing units. The game is played out on maps with various terrain features that permit players to expand, seize map control, and employ terrain features players use and abuse to their tactical advantage: expanding to new bases, seizing positional map control, and gaining line of sight or scouting information.

It’s an intensely challenging game that requires full attention from the moment the game starts to the end of the game. Matches rarely last longer than 20 minutes, so it’s easy to pick up and play. It requires at least some amount of practice and study to understand the potential strategies of your opponent and to be able to successfully execute your own strategies in the heat of battle. It requires multi-tasking: reading your opponent, controlling your units, and expanding your base all at the same time. The skill ceiling—the level at which players have “mastered” the game is unbelievably high, which leaves aspiring amateurs plenty of room to learn and grow. And it’s a game that constantly evolves: new strategies are developed, new reactions are tested, and the “metagame” gradually shifts. It’s this combination of factors that keeps the game fresh, challenging, engaging, and accessible.

But what is it that makes StarCraft 2 so exciting and fun to watch?

First and foremost, it’s foreknowledge. It’s like watching a thriller where you the viewer know what the bad guys are up to but the hero is still stumbling around in the dark. As a viewer, you get to see what both sides are up to, but neither player is certain what strategy their opponent might be pursuing.

Second, I think all the things that make StarCraft 2 fun to play also make it fun to watch. You can see players execute mind-boggling battlefield tactics controlling dozens of units at a time, sometimes individually! You can see players fake each other out, using misdirection and bluffing like stage magicians or poker pros. You can see which player is ahead and watch players try to claw their way back into the game. “Casters” commentate professional matches, offering their own reads on the players’ strategies (commonly known as “build orders” or just “builds”), calling out huge plays, and helping to explain to the casual viewer what he or she should be paying attention to. It has the excitement and energy of a great sporting event, and the more you know about the game, the more thrilling it is to watch it be played live.

Here are a few common strategies that you might see while playing or watching:

Cheese or Rush: One player goes for an early-game make-or-break rush that will either win or lose the game. If spotted in time, the defender usually comes out on top, but if the defender fails to spot the attack in time and doesn’t control his defenses perfectly, he or she may suffer a humiliatingly quick loss.

Proxy: A type of cheese that involves one player building an army-producing structure right next to his or her opponent’s base. This may allow the attacker to get a few army units into his opponent’s base before the defender has any suitable defenses prepared.

Harassment: This is typically an early- to mid-game strategy that involves trying to slow your opponent down by using small groups of units to destroy the opponent’s workers. Fewer workers means less economy. Less economy translates to smaller army and slower growth.

All-in: A tactic in which one player or the other just goes for it and throws everything that they have, knowing that if they don’t win in that moment, there is no coming back and they will lose. Can be a coin-toss moment.

Base-trade: Both players attack each other’s bases at the same time, but don’t have enough defenses to both attack and defend. It becomes a race to see which player can destroy all of the other player’s buildings first. These games are really exciting and unpredictable.

Timing attack: Equally-matched players will often have similarly sized armies and be progressing at similar rates. By executing a timing attack, a player looks for a brief window of opportunity where they have a very small unit or upgrade advantage over their opponent. The purpose of the timing attack usually isn’t to win the game right-out, but to widen the gap. Once the gap grows, it becomes difficult for one player to come back.

StarCraft 2 is not an easy game, nor is it a simple game. It has a steep learning curve even for the casual player. But it’s a deeply rewarding game, both for players and enthusiastic viewers alike.

If you’d like to try out the game, Blizzard offers a free version here. If you’d like to learn more about pro matches, John “TotalBiscuit” Bain has produced a great YouTube video on what to watch for.

Shannon Rampe (aka BeefMaster) is a member of PSIStorm Gaming, a long time StarCraft 2 player, and a writer of science fiction and fantasy. You can learn more about him at his blog. An earlier version of this article originally appeared here.