The Knee Smash (ストライキングニー, Striking Knee), also well-known in the fan community as The Knee and Knee of Justice (and a variety of other nicknames involving the term "knee"), is Captain Falcon's forward aerial in Super Smash Bros. Melee, Super Smash Bros. Brawl, Super Smash Bros. 4, and Super Smash Bros. Ultimate. It is an extremely powerful low-trajectory KO move often used at the end of Falcon's combos and for edge-guarding. It deals up to 18% damage in Melee, 19% in Brawl and SSB4 and 22% in Ultimate, and has electrical properties when sweetspotted. The name of the move is confirmed from his "Red" Smash trophy description in Melee, as well as that game's strategy guide.
In Super Smash Bros. Melee
The Knee Smash is considered among the best moves in the game due to its powerful horizontal knockback, where it can reliably KO under 100% in most stages. Many combos and strategies for Captain Falcon end with a Knee Smash. When the Knee Smash is at full power, it makes an excellent wall of pain, and can be accompanied by Captain Falcon's down aerial for a pseudo-Ken Combo.
The Knee Smash acts as a sex kick, with two main hitboxes. The initial hitbox (often called the "hard knee"), which lasts only a few frames, is Captain Falcon's primary KO move and is accompanied by an electric effect. The second hitbox (nicknamed the "soft knee" or "weak knee") lasts much longer, but lacks the damage and knockback of the initial hitbox. The attack acts as a semi-spike, albeit a weak one. Despite how seemingly weak it is, it has its uses. The player can use weak knee in comboing the entire cast with the most common combos being: weak knee into hard knee and weak knee into up air. Weak knee can also be used in edge guarding. By using his speed Captain Falcon can reach extremely far offstage and cover large areas with his knee. While sometimes the weak knee may not be enough to kill, it sets the player up to continue the edge guard.
Due to the Knee Smash's low ending lag, players can use it as a launcher into another aerial attack, often another Knee Smash. In addition, if a hard Knee Smash lands at 60% or so, players can occasionally follow it up with an aerial Falcon Punch in the deadly "Sacred Combo".
Assuming the move is properly timed and L-cancelled, the knee is one of the few moves that puts the attacker at a frame advantage when blocked. In other words, by the time the knee's landing animation is over and Falcon can act again, the blocking opponent will still be stuck in shieldstun, unable to act. This gives the knee immense utility in shield pressure scenarios, requiring the opponent to commit to a risky option should they wish to get out of their shield unharmed. The knee also supplements its utility in shield pressure with a high shield damage, removing nearly a quarter of the opponent's remaining shield upon being blocked.
In the PAL version of Melee, the late hit of the attack is weakened, dealing less knockback and halved damage. This nerf prevents Captain Falcon from following up with another hard knee, forcing him to follow up with an up aerial instead.
In Super Smash Bros. Brawl
In Brawl, the Knee Smash was nerfed. The initial hitbox was nerfed such that it needs to actually sweetspot on the knee itself for the familiar electric hit to occur, as the back hitbox simply has similar properties of NTSC Melee's late hit, while the late hit in Brawl acts like the PAL late hit in Melee, being even weaker than the clean hit's sourspot. In addition, the initial hitbox as a whole now only lasts one frame (instead of three) and its hitbox is significantly smaller (5.2u → 3u); as a result, it is much harder to land a hard Knee Smash, especially on smaller characters. Its hitlag has also been significantly increased (with the clean hit having a 1.2x hitlag multiplier), giving more time for the opponent to DI and SDI. On the upside, it can auto-cancel in a short hop due to Falcon being floatier, though on smaller characters, this trait can only be taken full advantage of if they aren't on the ground. Nonetheless, the Knee Smash remains among Falcon's strongest finishers (stronger than all of his smash attacks), sometimes used in an aerial combo in place of his up aerial.
The Knee Smash had its knockback reduced (24 (base), 100 (scaling) → 30/93) although it deals 1% more damage (18% → 19%), making it slightly stronger overall. The higher base knockback makes it safer on hit at lower percentages. However, the reduced scaling makes it slightly weaker at higher percentages. It also has 4 frames more ending lag (FAF 36 → 40) and 3 more frames landing lag (19 frames → 22) and the removal of L-canceling only further worsens its use as a landing option.
A late Knee Smash can trip a grounded opponent often (having a 30% trip chance), and is still useful for gimping recoveries like Falco's, albeit less so than in Melee. It can also be followed up with an up aerial or a sweet spotted Knee Smash if the opponent is read correctly.
It is notable that Captain Falcon has a true setup for a sweetspotted Knee Smash if he uses it after a aerial grab release on Wario and it can be used as a semi guaranteed followup on some other characters depending on their aerial grab release arc.
In Super Smash Bros. 4
The Knee Smash was nerfed once again, now having 6 more frames of ending lag (FAF 40 → 46) and 8 frames more landing lag (22 frames → 30). It also auto-cancels later (frame 35 → 42) meaning it can no longer auto-cancel in a short hop. As a result, it is no longer a practical approach option, and is somewhat riskier to use offstage. The late hit also has a lower bonus trip chance (30% → 20%) and the move's worse auto-cancel window and higher landing lag makes the move's tripping properties less useful.
Despite these nerfs, it sees much more use and effectiveness than in Brawl, largely due to the changes to hitstun canceling, which is now only possible after frame 40 with an air dodge or frame 45 with an aerial, while both hitstun canceling windows scale in proportion to the knockback sustained, rather than the frame 13 or 25 regardless of knockback windows that were in Brawl. These changes make combos into the Knee much more prevalent, such as the classic down aerial into Knee, especially in conjunction with Falcon's altered up aerial that now sends opponents vertically, and the significantly improved burst on his dash grab, that allows Falcon to carry a great deal of momentum into his down throw; the Knee as a fatal followup to these moves at mid-percentages is notably prevalent in competitive play, whether as part of a true combo or to punish an opponent air dodging out of those moves. The new rage mechanic naturally benefits the knee when Falcon is at higher percents which can allow it to KO up to around 20% sooner; it's not uncommon for Falcon to nab KOs below 50% from a combo into the Knee when he has a significant amount of Rage. At high damages however, about after 80-100% damage, even lower when he has significant Rage, it becomes difficult to land a Knee Smash again as the setups into it no longer work, requiring a good read or a precise punish to land it. Lastly, the reduction of hitlag and the weakening of SDI and DI make its power more consistent overall and it benefits from the inclusion of frame canceling.
In Super Smash Bros. Ultimate
The Knee Smash has been buffed in Ultimate, as the sweetspot is larger (3u → 3.75u), and it deals more damage (19% → 22%), albeit with its knockback scaling compensated (93 → 81). The move also now has 18 frames of landing lag (the same as in Melee when not L-canceled), increasing its safety and overall utility.