An auto-double is a two-hit normal activated by a single button press. It is aptly named, since you automatically get two hits for each button you press. It is only available while the combo system is activated, so auto-doubles cannot be done until an opener has started a combo, and you can only perform auto-doubles if your opponent is grounded.
To demonstrate what an auto-double looks like, the video does an opener and then immediately press HK. If you press the button early enough, two heavy kicks should connect. These normals often look pretty similar to your character’s regular normals. In fact, most of the time, an auto-double for a given strength will look like two of your character's close, far, or crouching normals for the button pressed, performed in rapid sequence.
An important thing to realize is how early the button must be pressed to trigger an auto-double. You must input the button before the active frames of the move preceding the auto-double are completed. You are effectively normal canceling a special move. It is quite common to input auto-doubles too slow at first, because pressing normal attacks during special moves is not common behavior for Street Fighter players. Just like how normals are special-cancelable, it really helps to think of specials as normal-cancelable within a combo sequence.
Each character has six auto-doubles, representing each of the six buttons in the game. It doesn’t matter if you are crouching or standing, the auto-double does not change. The damage done by an auto-double is equal to two hits of a normal move. So, for example, a heavy punch auto-double does two heavy punches worth of damage.
The difference between the different strengths of auto-doubles is important to understand. Heavy auto-doubles are slow and each hit delivers an audible “thunk” that will become easier to recognize as you play more matches. They are the easiest auto-doubles to recognize because of their slow, deliberate pace and often exaggerated animations. Medium auto-doubles are slightly faster than heavies with a slightly more subtle audio cue. To an untrained player, medium auto-doubles can look similar to heavy auto-doubles, especially in a fast-paced match. Light auto-doubles are very fast and the faster sound effect feels more like a soft “pat” for each hit. As a defender looking to combo break these attacks, you will have to learn how to recognize auto-doubles based on audio cues, visual cues, player habits, or most likely a combination of all three.
Lastly, auto-doubles only trigger if the previous move hit. This means if your opener missed, was blocked, or hit an airborne opponent, pressing a button for an auto-double will do nothing. This is one of Killer Instinct's primary option selects; after each opener, you should input a follow-up of your choosing regardless of whether you expect the opener to hit. If the opener hits, you will continue with your combo, and on block, the game will ignore your input. It's a good way to lessen the effort required for starting combos off openings in the mid-range, but be sure to switch between auto-double buttons on occasion, or you will become predictable.
So, what happens if you input another special move after this auto-double?
A linker is just a fancy name for a special move that continues the combo sequence. Linkers are special-canceled from normals in a combo sequence, such as the second hit of an auto-double, but not every special move can be used as a linker. Like openers, special moves that have been designated as linkers tend to keep the opponent grounded. In fact, if a special move is an opener, there’s a very good chance it can also be a linker, and vice versa. However, it is very important to note that a special move used as a linker has different properties and often different animations than the same special move used as an opener. Once you've entered the combo system, your buttons all do different things.
Like your special moves outside of a combo, and also like auto-doubles, linkers have three strengths. You perform a light linker and medium linker by inputting the special move with your light button or your medium button (punch or kick, depending on the linker’s command), as expected. However, you input a heavy linker by holding down either light or medium for a short period of time. This is because inputting a special move with a heavy button during a combo performs an ender. If you are a Street Fighter veteran, it may feel weird to actively avoid doing mid-combo special moves with your heavy buttons, since they are very frequently used for high-damage combos. However, in KI, you want to input special moves with HP and HK only when you're ready for your combo to be over.
Just like auto-doubles, light, medium, and heavy linkers all look different in some way, so they can be identified by the defense for combo breaking purposes. The most common way they are different is they will hit a different number of times: almost always 1 hit for light, 2 hits for medium, and 3 hits for heavy. Some linkers will hit a different number of times (such as 2, 3, and 4 times), but these cases are very rare. Some linkers increase the number of hits very subtly and quickly, which can be pretty difficult to tell apart in the middle of a match, while others will animate the character slowly winding up multiple versions of his special move back-to-back, which can be considerably easier to see. Heavy linkers are slow enough to usually be combo breakable on reaction, but it is virtually impossible to react to light (and even many medium) linkers during a high-paced KI match.
So, you’ve done an opener, then canceled the opener into an auto-double, then canceled the auto-double into a linker. What now?