Morph Ball
The Morph Ball is a movement system in the Metroid series that allows for Samus to pass through small tunnels and gaps where she otherwise wouldn't fit. In the Prime games, the Morph Ball's collision is in the shape of a sphere, and lends itself to very precise movement. It has a variety of use cases, which get expanded upon with the addition of various upgrades.
Metroid Prime
In the original Metroid Prime, Samus has immediate access to the Morph Ball, but will lose it during the Space Pirate Frigate escape sequence. Once on Tallon IV, she can retrieve it in Ruined Shrine, where she will keep it for the rest of the game.
Metroid Prime 2: Echoes
In Metroid Prime 2: Echoes, Samus can once again use the Morph Ball immediately, and won't lose it during the Item Loss sequence in Hive Chamber B. However, she will lose her Morph Ball Bombs and Boost Ball.
Kinetic Orb Cannons were introduced in Prime 2, which launch the Morph Ball to a designated spot when activated. These cannons give Samus Cannon Ball, which gives the Morph Ball total invulnerability and an aura that damages certain objects for 500,000 damage per frame. Additionally, it gives Samus Boost Ball if she doesn't already have it.
The Screw Attack upgrade in the original version of the game uses the same collision as the Morph Ball, which allows for Samus to enter Morph Ball Tunnels while in Screw Attack, leading to sequence breaks.
Metroid Prime 3: Corruption
The Morph Ball in Metroid Prime 3: Corruption has a unique mechanic that was first introduced in Corruption and subsequently added for the Trilogy versions of Prime 1 and Echoes. This mechanic is known as Spring Ball, and it allows for Samus to get as much height as a standard bomb jump by shaking the Wii Remote.
Usage in Speedruns
Wallcrawling
During a wallcrawl, it is sometimes important to transition into Morph Ball mode and lay a bomb if Samus gets stuck on the collision. This is sometimes dangerous however, as there are certain spots where morphing will result in an inescapable softlock, so great care needs to be taken to ensure that morphing won't kill the run.
Instant Morph/Unmorphing
Sometimes, you will be able to skip the unmorphing animation, saving time. This happens because when unmorphing, the game calculates where to place the camera, and if no possible position for the camera is found, it will skip the animation entirely.
Additionally in Echoes, it is possible to skip the morphing animation, which isn't possible in Prime 1 or Corruption.
Ceiling Warp
By morphing into the ceiling of a room that isn't currently transitioned to, it is possible to warp back in-bounds, provided there is enough momentum. This is most often done during the end of a wallcrawl when it is desirable to return in-bounds.
Morph Secret World
In some instances, such as the Frigate Wallcrawl or Exo Skip, it is possible to abuse the act of transitioning into the Morph Ball in order to reposition Samus out of bounds.
Infinite Speed
Mainly used for Wave/Sun, morphing inside certain points will get the Morph Ball stuck in such a way where it will be able to rotate at an exponential rate. By holding this for 50 seconds, the shell of the Morph Ball will disappear, and boosting will result in the morph ball being everywhere at once for at least a frame, allowing for the collection of items or the triggering of events in situations otherwise not possible. Once in Infinite Speed, Samus must activate a cutscene trigger, such as a save station or elevator, that will reposition her in order to return back to normal.