Single Wall Jumping (commonly abbreviated to SWJ) is a technique that allows the player to scale a single wall infinitely. Normally, a second wall is required to gain any height from a wall jump, but it's possible to interrupt the sideways momentum from a wall jump using upgrades. This can be done with either Morph Ball or Flash Shift.
Normally, the game forces Samus to move away from the wall when wall jumping regardless of the direction being held, even if none is being held at all. This can be bypassed by morphing immediately after wall jumping, which will cancel out the momentum if no direction is held.
To take advantage of this extra height, the player must unmorph at the peak of the jump and respin back towards the wall. The process can then be repeated until the top of the wall is reached.
Optimally, morph should be pressed the frame after wall jumping. If it's pressed any later, Samus will be pushed farther away from the wall and not gain as much height. Additionally, the stick should be neutral during the wall jump.
The best way to input this is to only tap towards the wall for the respin, like so:
This can easily be practiced in one of two places:
I morphed without a wall jump!
This means you didn't morph after the wall jump. Make sure the wall jump comes first!
I'm not gaining height on each wall jump!
There are multiple reasons this can happen:
The easiest to recognize is morphing too late, as Samus will jump away from the wall at a 45 degree angle. You want her to go more up, closer to 20 degrees. Other than that, generally just try to tighten up your inputs.
I keep immediately dropping down after I wall jump!
If this is happening, you've triggered the Gravity Ball glitch. While this glitch is active, you cannot hold jump to gain height while in morph ball. This can easily be fixed by unmorphing while touching the ground. See more here.
Single Wall Jumping with Flash is as simple as flashing back towards the wall with each jump, as it immediately interrupts your momentum. Aim to do the flash at the apex of your jump to gain the most height. You can wall jump to stall the recharge timer and repeat the process forever, like so:
It's also possible to Flash Chain a SWJ to speed up the process. This requires very quick inputs and can be done like so:
To optimize this, it's important to flash as late as possible to gain the most height. You want to use all of the flash chain timer's leniency, so try to internalize the limits on flash chaining.