The Jab Fake or jab cancel is a technique available in Super Smash Bros. Melee and Super Smash Bros. Brawl. It is considered a far more potent strategy in Brawl. It is used to perform quick combos at close ranges.
The Jab Fake can be performed by any character with a multiple-attack Neutral A Jab Combo. At any point in the combo, before reaching the "final" hit, Shield or Crouch can be tapped to cancel the combo and allow the character to perform other A moves without continuing the combo. Only Crouch works in Melee, due to the longer time it takes a Shield to drop. Jab Faking is beneficial because the final hit of a 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. Jab Faking 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 Fake can 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. This is especially useful against a Shield-grabbing opponent, as most jabs are faster than grabs, allowing the Jab-Faking character to attack a Shield, Jab when the shielding character attempts to grab, and segue into a grab of their own. Jab Fakes 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's Down Smash Paralyzer and Luigi's Super Jump Punch Sweetspot, the Fire Jump Punch.
There is also a way to be able to continuously time the jab fake by holding down and back after your 1st jab, or before your 1st jab (if you are crouched). You will only have to time your jabs instead of jab, down, jab, jab. - voorhese
The Jab Fake 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.