Fox (SSBM)/Neutral attack/Hit 1

Overview

Frame 3 hitboxes of Fox's jab.

Fox does a quick punch. This is most commonly referred to as Fox's jab. It is his second quickest attack next to shine, as the hitbox activates on the second frame. It is useful for it's quick start-up, low lag, decent range, and combo potential. It can, however, be susceptible to out of shield punishes and crouch cancel counters as well as strong ground attacks and disjointed attacks which out-prioritize it.

Neutral

In neutral, jab is in many ways out-shined by Fox's down special, which is faster, cannot be crouch canceled, has less lag, and does more knockback (until very high percents). Jab is still useful though because of its range in front of Fox and the different potential follow-up options. The following is a full examination of jab's strengths and weaknesses:

Strengths

Quick start-up: Though the hitbox comes out on frame 2, Fox only extends his fist on frame 3. Even so, the attack is Fox's second fastest attack behind shine. The quick speed helps to hit first in close encounters which your opponent may choose a slower option. Also, at this speed it is impossible to react with a defensive option or DI on reaction. The opponent must be prepared for the attack beforehand to respond appropriately. This fact is vital to a move which is made useless by proper DI.
Low lag: Only 12 frames of cooldown lag leaves little time to punish a whiffed jab, especially because of how Fox lunges his body forward and pulls himself back. For this reason it can be a useful move to use when concerned about an opponent trying to evade and punish your hit.
Good range: A key advantage over using shine, jab has covers a solid area in front of Fox.
Follow-up potential: A weak hit with a launch angle of 70 results several strong follow-up possibilities as discussed in the section below. This is also sometimes an advantage over using shine, because Fox sometimes has stronger follow-up options from a jab than a shine. For example, jab can lead to an upsmash on Jigglypuff, but a shine cannot.

Weaknesses

Weak hitbox: Low knockback means it is easily crouch canceled or even only ASDI down, which is a glaring flaw against grounded opponents with quick crouch cancel counters.
Short active hitbox: The hitbox is only active for 2 frames, which makes it useless for walling out opponents.
Low priority: Opposing ground attacks of 13% damage or more will override the jab hitbox (most smash attacks and some tilts).
Small disjoint: Jab is unlikely to do better than trade with most attacks, and attacks with high disjoint such as Marth's fair will almost surely override it.

The issues with jab is that the hitbox is not active long, and the threat of crouch canceling or ASDI down limits it's use, and shine fulfills many of it's potential purposes. The only advantages it has over shine in neutral is range in front and better follow-ups against some characters (Jigglypuff, for example).

It is most safe to use against an airborne opponent because the inability for the opponent to crouch cancel eliminates a key weakness.

Shielded

Jab (unstaled) has a frame disadvantage of 9 to 10 on connecting with full shield according to the following calculation:

Shieldstun (7) - Hitlag (4) - Total Duration (15) + Hitbox (2 to 3) = -9 to -10

From a mathematical standpoint, jab can be punished out of shield by the average grab which is effective on frame 7. However, this dependent on the reaction time, timing, and out of shield options of the shielder as well as the spacing and use of jab 2 of the attacker. The window for a shield grab is only a few frames, and in some cases it is possible for the attacker to space the jab out of range of the shielder's out of shield options. Immediate use of jab 2 also leaves the shielder with only as few as 3 frames, which if disregarded may catch the shielder's counter option. However, jab 2 is more easily punished than jab 1 if shield is held and use of auto-smash DI down can render even a connected hit as punishable.

Overall, jab is generally safe from shield punishes when performed from it's maximum range, but it is a a risky choice within shield grab range.

Crouch Canceled

Jab is very susceptible to crouch canceling at even high percents, and even ASDI (auto-smash direction influence) down is dangerous. Depending on the situation, the defender will have about a 10 to 15 frame advantage, with access to their full moveset (in contrast to the limited options in shield). This means more characters will have faster attacks with more range to counter than from in shield. A poorly spaced jab can be crouch cancel countered easily by the full cast, and even a well-spaced jab may not be safe against most characters. It is recommended to use jab sparingly against grounded opponents for this reason. One must consider the risk-reward for connecting the jab as well as if it will be anticipated. Jab is too quick to crouch cancel on reaction, so if the opponent is not prepared to crouch cancel, the jab will hit safely. Consider using moves that cannot be crouch canceled (shine, grab, dair) frequently to condition the opponent to not try crouch canceling.