What exactly does "Stick Sensitivity" do in Smash Ultimate custom controls?

Stick Sensitivity is a deceptively named control option. It has nothing to do with run/walk/dash sensitivity, platform drops, stick distance/movement/precision, or anything like that.

Stick Sensitivity only controls the frame window that differentiates between a Smash and a Tilt.

• Low stick sensitivity = easier Tilts

• High stick sensitivity = easier Smashes

After you "stick flick" your left analog, if you press "A" you will input a directional attack. Stick Sensitivity adjusts the frame window that determines if that attack will be a Smash or Tilt.

• Press "A" in the next X frames = Smash

• Press "A" after those X frames = Tilt

Lengthening this frame window results in Smashes being easier to input and Tilts more deliberate.

Stick Sensitivity Frames
========================
        | L | N | H |
F-Smash | 5 | 6 | 7 |
U-Smash | 3 | 4 | 5 |
D-Smash | 3 | 4 | 5 |

Your optimal Stick Sensitivity will primarily depend on whether you set your C-Stick / Right Analog to input Smash or Tilt. You will want to set Stick Sensitivity so that the opposite class of attacks is easier to perform with the Left Analog Stick + Attack button.

• If you use Smash Stick, make A-button Tilts easier to perform with Low Sensitivity
• If you use Tilt Stick, make A-button Smashes easier to perform with High Sensitivity

Set a lower sensitivity if you find it hard to Tilt with the A-button.

If you primarily Smash with the A+B shortcut, set Low sensitivity for easier Tilts.

Setting sensitivity to Low is the closest you can currently get to disabling Smash attack input from your A-button and Left Stick. You unfortunately cannot completely turn off Smash attacks.

References:

• https://smashboards.com/threads/all-about-stick-sensitivity.466696/
• https://www.reddit.com/r/smashbros/comments/aafl58/stick_sensitivity_is_a_lie_read_for_truth/

In Japanese, the Stick Sensitivity option is titled はじき入力 meaning "Flick Input".

The options are:

• はじき入力 "Flick Input".
  • 出にくい "Hard to input" (Low)
  • 普通 "Normal"
  • 出やすい "Easy to input" (High)

This seems to indicate that "Stick Sensitivity" is a bad translation of the Japanese original. It's much easier to understand "easy" and "hard" settings for "Flick Input" aka Smash Attacks.

Source - user Logo121 on Reddit