Sonic the Hedgehog 2 game mechanics & glitches

Terminology

Spindashing: Each additional button press in a spindash gets you more speed, up to a maximum of six. Sometimes having a specific amount of taps (less than six) gives you the right speed to get through a certain area quickly or easily.

Universal timer: There's two different timing properties of moving objects in this game. The universal timer is where an object starts moving the instant you start the level, and will therefore be in a certain position at a certain time, every time.

Camera timer: The other kind is this one, which only starts moving once it's brought onto the screen (or near the screen - when the object is first rendered, in any case). Knowing which objects have which timing properties is important to good times when you have to deal with them.

Ramp jump: Sonic's jump speed and height are dependent on both your speed going into the jump and the slope of the ground you jump off. Running up a slope causes your jump to become much higher, while running down a slope will make you jump quite flat to the ground, gaining speed. This applies even if the slope is very small.

Loop jump: Similar concept, but inside a loop. If you're moving through the loop from left to right, a loop jump is from the top-left to the bottom-left. It's kinda finicky because if you land at different angles you get different speeds, but in the best case it is faster, and it's also a very convenient way to switch from rolling to running.

Path switcher: This is an invisible item in the game that allows the levels to work as structured. There are three different levels of background/foreground and Sonic is always in one of them at any one time, and these items seamlessly switch Sonic from background to foreground and vice versa. This is especially necessary for things like loops to work, because if you didn't have them they would just be solid all the time and you wouldn't be able to enter it. Knowing where these are and how they work is key to some tricks in this game.

Speed cap: Oddly for a game designed and marketed around speed, there are certain situations where the game will actually limit your speed. If you hold the direction you're moving while you're in the air, and you're above the speed cap, the game will knock your speed back to the speed cap. This can be circumvented just by not pressing right, but there are certain situations where the speed cap is actually beneficial, too. This is specific to Sonic/Tails play: if you play as Knuckles, the speed cap is removed.

Superglide: As Knuckles, simply jump on an object (an enemy or an item box) and glide just before you hit it. This causes you to translate your falling speed upwards, and since you're gliding you have a much lower rate of acceleration towards the ground. In layman's terms, you end up a lot higher than a standard jump.

Zip: A zip occurs when Sonic is stuck in a wall somehow and presses left or right. Since collision detection is designed to stop you from going into a wall in the first place, pushing left against a wall makes the game push you back to the right so you aren't embedded in it. As a result, when you're already inside a wall holding left or right causes you to travel in the opposite direction much faster than you can attain by normal play.

Level wrap: This is what happens when a zip causes you to bypass the left edge of the screen. The game treats your X-coordinate in memory as a number less than 0 - but position coordinates are unsigned, so instead of a negative number it's a VERY large positive one - which correlates to the end of the stage. This requires some very specific circumstances depending on the level, but it can be used to skip Metropolis 2, as well as Metropolis 3 if playing as Sonic and Tails.

Screen wrap: Similar concept, but using Y-coordinates instead. If you can move off the top of the screen, the game thinks you're all the way at the bottom. This only works in certain levels which wrap around themselves infinitely - which in Sonic 2, is the three Metropolis stages only.

TASvideos has a much more in-depth analysis, mostly dealing with the way the engine works. It can be found at http://tasvideos.org/GameResources/Genesis/SonicTheHedgehog.html (this covers all three games in the classic series).