Knee Smash

Not to be confused with the first hit of Solid Snake's forward tilt, which can be used to perform the Knee Lock.

The Knee, is Captain Falcon's forward aerial in Melee and Brawl.
(In the original, it was what is now Falcon's N-Air, and his N-Air in SSB64 was a sex kick.)

It is an extremely powerful killer that sends the enemy at a low trajectory.
This move is considered by some to be the best move in the game, due to its short lag time and powerful knockback ability.
Most combos and strategies end with a knee, which deals up to 18% in Melee and 20% in Brawl, under NORMAL circumstances (ie NOT Giant status).
The Knee has two main hitboxes: the initial, much more powerful hitbox, and the second, much weaker hitbox.
The initial hitbox (only a few frames in length) is the primary killer in Captain Falcon's arsenal. It is accompanied by an electrical pulse.
The second hitbox lasts much longer, but it lacks most all the power of the initial hitbox.
Its knockback is minimal, so many use it as a precursor to a successful follow-up true Knee.
In Brawl, the knee is much harder to land, and its knockback power has decreased significantly.
If a true Knee is landed, the hitstun lasts about a second, much longer than in Melee.

Interestingly, Ganondorf's aerial forward A is not a knee, instead being more of an arcing downward punch (in both Melee and Brawl).