The Knee Smash, also well known as The Knee or the Knee of Justice, (and a variety of other nicknames involving the term "knee"), is Captain Falcon's forward aerial in Super Smash Bros. Melee and Brawl, as confirmed by his "Red" Smash trophy description in the former. 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 and 19% in Brawl, and has electrical properties.
 In Super Smash Bros Melee
The Knee Smash is considered among the best moves in the game due to its fast start-up lag and very powerful horizontal knockback (where it can reliably KO under 100%). 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 has two main hitboxes. The initial hitbox, which lasts only a few frames, is Captain Falcon's primary KO move and is accompanied by an electrical effect. This hitbox is considered the attack's sweet spot, but is actually closer to a sex kick in function. The second hitbox lasts much longer, and lacks the damage and knockback of the sweet spot. The attack, however, acts as a semi-spike, albeit a very weak one.
Due to the Knee Smash's low ending lag, players can use it as a launcher into another aerial attack, often a sweet spotted Knee Smash. In addition, if sweet spotted at 60% or so, players can occasionally follow it up with an aerial Falcon Punch in the deadly "Sacred Combo".
 In Super Smash Bros. Brawl
In Brawl, the Knee Smash was nerfed. The sweet spot became much harder to land, as the back hitbox is a slightly stronger version of the sourspot, while the sweet spot's hitbox only lasts 1 frame, and its knockback was decreased slightly. However, its hitlag lasts about a second, much longer than in Melee. Nonetheless it remains among Falcon's strongest finishers, sometimes used in an aerial combo in place of his up aerial.
A sour spotted Knee Smash can trip a grounded opponent often, and the sour spot hitbox is still useful for gimping recoveries like Falco's. It can also be followed up with an uair 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 if he uses it after a grab air release on Wario, and a grab ground release on Ness.
 Skull Crusher
Though Ganondorf was a clone of Captain Falcon in Melee, his forward aerial is not a knee, but is instead an arcing downward punch resembling a faster version of Mario's fair in both Melee and Brawl, nicknamed the "Skull Crusher". In Melee, it has the same number of start-up frames as the Knee Smash, but has less knockback (though still strong enough to reliably KO under 120%), and longer ending and landing lag. Unlike the Knee though, it has only one hitbox in Melee, with longer reach, allowing the move to be landed easier. Due to its rather fast speed for such a strong attack (that has no sour spot hitboxes), and its great reach, the Skull Crusher became a pivotal move in Ganondorf's metagame.
In Brawl, similar to the Knee Smash, the move was nerfed, despite a slight increase in its knockback dealt. It has 3 frames slower start-up, along with drastically longer ending and landing lag (which is made worse by the removal of L-Cancelling). It was also given a sourspot hitbox that covers Ganondorf's arm, making the move require spacing to land properly. In comparison to the Knee Smash, there is much less difference in damage and knockback between its two hitboxes compared to the 3 hitboxes of Captain Falcon's knee (whereas the Skull Crusher's sour spot can still KO under 150%, while the two sour spot hitboxes of the Knee Smash deal minimal damage and knockback).
 See also