PSI Magnet (サイマグネット, Saimagunetto; called Psychic-Magnet in Super Smash Bros.) is the down special move for Ness and Lucas. It absorbs certain projectile attacks (mostly energy-based rather than physical objects). Upon absorption, the attack lowers the user's percent (or replenishes their health in Stamina Mode). The user cannot be healed to a percent lower than zero or higher than their starting HP, usually 150HP (Stamina Mode). The move also differs slightly between the two characters that use it.
Ness' PSI Magnet is formed centered around his body, covering his entire body with it. When released, Ness's Magnet pushes enemies and some items (such as Snake's Grenades) away from his body. In Super Smash Bros., the move has very high starting lag, making it nearly useless; in return though the crowd releases very enthusiastic cheers for its successful use. In Melee, it is now blue instead of green and is much faster, making it more reliable. In Brawl, the color changes again, from blue to a more purplish color. This move recovers about 1.6x (or one and two thirds) the damage the projectile would have dealt normally.
Lucas' PSI Magnet is formed centered around his outstretched hand, and thus the magnet doesn't fully cover the area behind him. However, Lucas will instantly turn around when hit from the back, which means the field essentially covers his whole body as well. Lucas's version of this attack also has a hitbox on his hand when released, which deals 9% damage and moderately strong horizontal knockback. This hitbox however doesn't have much practical use, due to the delayed nature of it and its extremely small hitbox making it very difficult to land, while not offering much reward when landing it. Additionally, Lucas recovers more percentage than Ness when he absorbs a projectile. This move recovers 2.5x the damage the projectile would have dealt normally, a 56% improvement over Ness'.
 Tactical uses
When used in the air, this move will cancel any vertical/falling momentum temporarily. This allows for this move to be used as a pseudo-SHFFL if used immediately after a jump, or a pseudo-wavedash if the player activates the move at a downward diagonal angle. Using C-sticking special moves control scheme for the Gamecube Controller, this move, used right after PK Fire for Lucas, will allow the wavedash to work much more easily.
When a projectile is absorbed, if the player taps the control stick/pad (depending on controller) in a direction, Lucas or Ness will either roll in that direction, sidestepping, or jumping without having to release the attack first.
PSI Magnet first appeared in Mother and has appeared in later Mother games since. However, in the Mother series, neither Ness nor Lucas learn the move. In all three of the Mother games, the move involved the player absorbing an enemy's Psychic Points (or PP) and adding it to that player's PP (takes 10 PP in Mother and 2-8 PP in EarthBound and Mother 3). There was also a move in the Mother games called "Lifeup", which was a psychic move that Ness and Lucas could use to recover a player's Health Points (or HP). Also, PSI Magnet can absorb energy projectiles from other players in the Super Smash Bros. games, but since Ness and Lucas don't have limited PP in the Super Smash Bros. games, it instead recovers a small amount of health. PSI Magnet recovering damage could also be a reference to Lifeup. 
 Description from the Melee instruction booklet
Absorb enemy energy attacks and recover health.
 Description from the Brawl instruction booklet
Absorb projectiles to reduce Lucas's damage percentage a corresponding amount.
 See also