The jab cancel or jab fake is a technique available in all five Smash games. It is used to perform quick combos at close ranges, and is considered a far more potent strategy in Brawl, Smash 4, and Ultimate.
The jab cancel can be performed by any character with a multiple-attack neutral attack jab combo. At any point in the combo, before reaching the "final" hit, the player can shield, crouch, or quickly turn around and back to cancel the combo and allow the character to perform other attack moves without continuing the combo. Shielding to cancel the jab can be performed in Melee, but due to the longer time it takes for a shield to drop, it is seldom used.
Jab cancelling is beneficial because the final hit of a jab combo is usually either a medium-strength attack that knocks the opponent away or a multi-hit attack that is easily DI'ed out of. It also allows the player to take advantage of the hit-stun and low knockback of the first few attacks in a jab combo to continue to attack the opponent, rather than sending them away uselessly (the last hit tends not to have enough knockback to KO) or pointlessly jabbing repeatedly at nothing after an opponent DIs out of a multi-hit attack.
The jab cancel can also be used to tack extra damage on the beginning of combos with quick moves, such as tilts. It can also be used to open the opponent up for a grab (not to get confused with jab grab). This is especially useful against a shield grabbing opponent, as most jabs are faster than grabs, allowing the jab cancelling character to attack a shield, jab when the shielding character attempts to grab, and segue into a grab of their own. Jab cancels can also be used to open the opponent up to attacks that are often difficult to land on their own. Because jabs tend to be very quick and often have a good range and low knockback (or even knockback that moves the opponent closer), they are a great way to land difficult moves. Two of the best examples of this are Zero Suit Samus' down smash or Paralyzer and Luigi's Super Jump Punch sweetspot, the Fire Jump Punch.
Main article: Jab lock
The jab cancel can also be used to repeatedly do the starting hits of a jab combo against an opponent. This is most potent with jabs that move the character slightly forward, like Sheik's. This is performed by jabbing, canceling the jab with a jab fake, and starting another jab combo. This can be done repeatedly until the opponent DI's out or are at a high enough percentage to be knocked out of it.