Executed by pressing forward, down, down-forward and kick, Alex jumps up launching his knee forward. If it connects with an opponent, the knee catches them in the chest and as both fighters fall to earth, Alex stays on top and keeps his knee in his opponent's chest, driving them into the ground. The forward distance Alex travels during the upward jump is determined by the strength of the kick button pressed (with the light kick version being the shortest distance and the EX version being the longest).
This move was likely inspired by the high knee strike used by professional wrestlers Harley Race and Triple H. The biggest difference between the wrestling maneuver and the version Alex uses is that he follows them down to the ground after the initial strike. Alex does have the aforementioned high knee strike as his universal overhead (MP+MK) in Street Fighter III: 3rd Strike.
The attack cannot connect with a grounded opponent, unless the EX Special version is used. All versions of this move have a rather large hitbox and rely primararily on the hitbox to outprioritize aerial attacks rather than invincible frames. Moves with good hitboxes of their own, such as Urien's jumping HK etc can beat this move clean depending upon the angle of both moves. The Air Knee Smash (along with the Slash Elbow) is considered a secondary move for Alex and thus; unlike his signature moves, the Flash Chop or Power Bomb, it does comparatively low damage and needs EX meter to be effective. The EX version does inflict a good amount of Stun, however.
The non-EX versions of this move have a 'grab' hitbox. Hence they cannot be parried in the air. The EX version on the other hand has a small hitbox in front of it that can connect with grounded opponents. It has a miniscule amount of invincible frames on startup, thus can be used as a reversal uppercut. With timing, it can go through air-to-ground projectiles such as Akuma's Zanku Hadoken or Ibuki's Kunais. Functionally it can be seen as similar to Dan's Koryuken. The EX special version has a 'strike' hitbox and therefore can be parried on the ground or in the air.