As shown above, the size of Samus's hitbox varies based on the condition of her sprite. The most important detail is that the height of her down-aiming and spinjump hitboxes is considerably shorter than the height of her standing and normal jump hitboxes.
When jumping to any platform (regardless of whether the platform is above, below, or level with Samus) in the spin-jumping or aimed-down position, it is possible to land earlier by extending your hitbox to full length. This is done by either pressing angle up/down or shooting.
Regardless of the platform's position, when Samus is jumping to any platform in a spinjump or while aiming down, it is possible for Samus to land earlier on the platform by extending her hitbox to its full length, which can be done by pressing either angle up, angle down, or firing a weapon.
When performing a spinjump, by pressing angle up, angle down, or firing a weapon once directly above a platform, Samus will break out of her spin and immediately land on the platform. This allows her to land on platforms earlier than normal and is known as ledgegrabbing.
Similarly, Samus can land on platforms that are above her even earlier by entering the down-aiming position during the ascent of the jump, which brings the bottom of her hitbox up slightly, and then extending her hitbox to normal length once level with the platform, which can be done simply by moving forward to break out of the down-aiming position. This is known as downgrabbing.
Downgrabs are considered more difficult to efficiently execute compared to ledgegrabs. Although performing downgrabs as described on TASVideos is the fastest option, it is extremely difficult to consistently perform them. It is considered much easier and more reliable to perform ledgegrabs where angle up or angle down is pressed once Samus has ascended slightly above the location of the platform.
Considering whether to downgrab or ledgegrab a platform requires analyzing the risk versus the reward involved. Similarly, considering which speed to grab the platform at requires a similar analysis. Although you can save additional frames by performing a more efficient ledgegrab onto a platform, and although there will be dozens of opportunities over the course of a speedrun to perform efficient ledgegrabs, there is always the risk that if you perform the ledgegrab inputs too early, you will miss the platform and fall.
Certain techniques such as the Full Halfie may require stronger consideration towards performing a downgrab. In this case, you would be potentially providing yourself more time with which to activate the shinespark.
By running straight off of a ledge and pressing down to enter the down-aiming position, it is possible for Samus to clear gaps between tiles. The size of the gap she will be able to clear is primarily dependent on her horizontal speed at the time of leaving the ledge.
Depending on your speed and the size of the gap in question, you may need to remain in the down-aiming position before releasing a forward direction to clear it.
As downgrabs indicate, Samus will normally break out of her down-aiming posture any time a forward direction is pressed on the directional pad; however, it is possible to move forward while falling in the down-aiming position by pressing down and moving your thumb to down+back. (commonly referred to as a "diagonal" on the directional pad)
Although such input is counter-intuitive, there are multiple instances where downbacking allows Samus to squeeze through certain gaps without landing on platforms due to her compressed hitbox, thus maintaining her vertical speed.
You are normally unable to downback after a jump or walljump; however, either taking damage (without damage boosting) or unmorphing will allow you to do so.
Examples of downbacking can be observed in the video below at 0:01 after running off the starting ledge, at 0:04 after unmorphing to smoothly clear the two blocks, and at 0:06 to pass by the last ledge.
Samus will begin her jump eight pixels higher than normal if she is crouching before she jumps, effectively increasing jump height by 8 pixels and allowing her to grab onto platforms she may otherwise not have been able to grab onto.
The eight pixels will not be granted if Samus's arm cannon is not in the neutral position when attempting to crouch jump, in other words, holding angle up or down and doing a crouch jump will NOT increase jump height.
The video below is an example of a distance between two platforms that is surmounted by assistance from a crouch jump and a downgrab.
The following is a modification of the description provided by TASVideos:
When spinjumping forward, the game checks the tiles immediately behind Samus to see if she can wall jump, starting with the uppermost tile and moving down. If any solid tiles are found, the game stops checking and allows Samus to walljump.
The game considers Samus as touching any tiles checked, allowing her to trigger items and doors earlier by spinjumping and turning around right as the tile is within walljump check range. If an item that is to be collected requires Samus to turn around immediately after collecting it, using this technique can reach the item a bit faster.
This can also be utilized when entering doors that do not require momentum in the direction they are entered, such as rooms containing bosses.
It is also possible to check doors by having Samus collide with a ceiling or by having Samus break out of spin by pressing down and the direction opposite of forward. The latter is observable at the end of every Bomb Torizo skip performed in recent tool assisted speedruns, as it provides the fastest method with which to escape its chamber. These methods preserve horizontal momentum.
The maximum distance of a remote trigger pickup depends on Samus's velocity at the moment of the walljump check and can go as high as eight pixels away from the target.
Also known as "corner boost" and can be thought of as the opposite of ledgegrabbing, edge boosting is a technique that can be used to fall faster than normal. It is primarily performed in tool assisted speedruns.
When falling alongside a surface that ends, by aiming down and then immediately pressing left or right on the directional pad, Samus's position will be shifted a few pixels downward. In the case of pressing the directional pad towards the surface, this will allow Samus to pass under the surface faster.
This is possible because Samus's down-aiming hitbox has about half the height of her normal hitbox, and by pressing left or right, Samus restores her hitbox to full size and is immediately pushed down out of the boundaries of the surface she is colliding with.
It is also possible to perform an edge boost by breaking a spinjump when Samus is directly under an edge. This is more feasible for humans without tools and can be utilized in rooms such as the Green Pirates Shaft.
This technique is the most effective at the beginning of a jump or fall. At Samus's full downward speed, she may fail to be pushed down if she falls too fast for her hitbox to extend towards the boundaries of a surface.
Pasky - Works with snes9x v1.43 and snes9x v1.51 "rr" versions. (Right click -> "Save page as" -> save as a .lua file)
Total - Various "Super Hitbox" viewers for lsnes and Bizhawk.
Super Metroid Hack Wiki - Many different custom viewers for various emulators.