The battle is close, your allies need help, but your spells are cooling and your basic attack won't do enough. Don't just stand around, your hero has a body -- get in there and put it to use! Blocking is one of the most neglected, yet useful, techniques in combat. If you've been practicing your lane control, by now you should know that you can jump in front of your creeps in order to slow them down. The exact same principle can be applied to enemy heroes.
Note: If you haven't read about how to block units yet, go check it out in the Lane Control article before reading on.
Introduction
Blocking is an awesome trick to have up your sleeve on both offense and defense. It can prevent an enemy from escaping the fight or save an ally that is getting chased down. The best part is that you can do it with practically any unit in the game! It doesn't matter what hero you have, how much mana you have left, how long your spells have left to cool -- you can always step in and block someone.
The most common to block is to prevent someone's escape. If your allies are nearby, but not quite in range to help, a couple of seconds of blocking can keep your target around until they arrive. For example:
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Blocking to Save Allies
Blocking to save your allies is far more dangerous, if your team already is at a disadvantage in the fight, getting in close with your opponents to block them will often mean that you die in the process. However, it can certainly be worth it to trade your own support hero for the escape of your team's carry, or sometimes it's a question of if both of you die or one goes back to block so that the other lives. Blocking is at its most effective for saving your teammates against melee-range chasing heroes, like Lycanthrope and Clockwerk. A morphed Lycan is certainly fast, but if you can get between him and his target, you can often prevent him from getting the kill.
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Blocking with Blink
Heroes with Blink have a special advantage, in that they can almostly instantly jump anywhere they want on the battlefield to get in somebody's way. For instance, a Sandking who's Burrowstrike is cooling, but has Blink ready doesn't haven't to sit around and wait for Burrow to cool to get back into the fight. He can Blink in front of his target and slow them down until Burrow comes up. Blinking directly in front of someone is especially useful for Antimage, as it will block them and allow him to land several extra hits.
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Blocking with Illusions and Summons
Blocking is not only a technique for every hero, but also a great way to get extra mileage out of illusion runes and summons. You can get a lot of extra attacks off on an enemy by ordering your real hero to attack them, then switching to controlling your illusion or summon and blocking that enemy. This is a rather advanced technique, as it requires you to be able to maintain a block with the illusion, while not losing track of your actual hero. Don't worry if you can't get manage it on the first couple of tries, but it's a useful trick and worth putting some practice into.
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Mindgames
It's also worth noting that a lot of mindgames go into blocking someone. While sometimes there's only one direction that they can go to escape and it's not hard to cut them off, this will not always be the case. Often in order to block someone, you'll have to make a guess on which way they are going to go. At that point, blocking becomes a mindgame, and there can be a huge payoff if you predict your opponent well. For example, check out this clip where slahser's Warlock takes down KuroKy's Shadowfiend by anticipating the direction he'll run when he reaches the trees.
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Summary
While blocking can be hard to get the hang of at first, especially when using other units to do it while still controlling your hero or trying to predict your enemies, it's well-worth learning. Besides, since you can do it with practically any unit in the game, you'll never be short of opportunities to practice it. Best of luck!
