The only time just tapping block will result in a block is with either luck, or supreme timing, generally stemming from patternistic counters. Generally, you gotta hold it at least somewhat.
This is the def jab section from UMK:
All Defense Jabs:
To explain the tactic, the player is semi-permanently holding Block after starting the cycling of either high or low jabs, and in many cases is also holding down (to prevent being swept or the few low hitting moves in the game) and/or back (to prevent being thrown. This is an "all defense" attack pattern. We can call them "Def Jabs" for short, instead of Kara, because they aren't the same thing. During this animation, sweeps are blocked from a standing position since the character is locked that way, and they cannot perform moves like a Teleport Punch, Spear, Spin, Wave Punch, etc, but they can do charge moves like the Sai Shot, Cannon Ball, Bike Kick, Ground Pound, and simultaneous button press moves like the Slide or Leg Grab. Again, this is if they are committing to holding Block.
The player often times cannot jump over the jabbing character, even if they don't have a particularly good anti air move that can be done quickly after releasing block, like Kabal's Spin for example, but the active factor is the that the punches are constantly throwing a wall of collision in front of them in short intervals, so in order to clear the jabs you have to be far enough away on the jump to not be hit on the way up, but you will usually land so you are between the character and their jab collision, and get pushed back into the jabs anyway.
The best thing to do against the glitch jabs is repeatedly sweep the tip of the jabs from as far away as possible to make the character block and enough of this will cause them to change things up. Remember, during a sweep the collision box of the character is changed from being a thin rectangular box in the center of the sprite, into a box that is the entire surface area of the entire sprite's max dimensions. Players most often notice this while being swept during a projectile or a roundhouse when the sweep has "extra range." It's always like that, it's just more blatant the further the reach.
You can try to Kara jab against them to just stop the animation and guarantee you punishment safety, but Vs a character like the Female Ninjas, or Kabal, it is difficult to get in close enough without just blocking yourself and not interrupting the Def Jabs. Remember, vs any character, your advance can be vulnerable, so you have to attack with the anticipation you may be countered. But just as you are vulnerable, so are they, and more so because their best reward also has the highest risk.
Vs a character like Kabal you also have to worry about the spin, which can beat out a sweep attempt before it happens. After one or two of these sweep attempts, they will most likely attempt a spin on a wakeup whether it's blockable or not due to the anticipation of the player being frustrated at not being able to land positive contact at all and only achieving chip damage. Most importantly, always be ready to block because the Spin can benefit from random usage in this situation more than usual. Kabal is one of the most commonly used characters to abuse this tactic because the suddenness of the Spin complements it so well.
You can also just "stand there and wait" until your opponent gives up and comes in and try counter, however if they are ahead on energy, they will most likely let the time run as long as possible if they are fully committed to the tactic, since you are forced to attack. Your goal should be to get ahead on damage even just slightly to force stupid mistakes. Running in and trying to counter kara jab, or doing the tactic yourself generally leads to minimal success, so the max distance sweeps are you best bet. A blocked sweep from max distance is the best bet.
Getting in closer with ducking LKs works better with Sonya, Kung Lao, perhaps the Robots or even Sheeva, but you really want to create distance to prevent from being punished. A character like Mileena can punish a blocked sweep with a Roll for example, and a Teleporter can come out on the other side and pressure you to the wall.
You can fake a rush and move back, to bait a walk but they will often jump away with a fireball, which is a good time to connect something like a teleport punch or rush in and hit a HK if Kabal does an air fireball, but watch for the Spin after if you rush on his landing. If you jump back, also watch for the Spin while you're in the air, so it's better to use a character like Smoke or Ermac if you're going to attempt jump back strategies so you can teleport behind a Spin, and also Teleport Punch (and punish) Kabal jump backs after Def Jabs are abandoned periodically to mix it up.
A well timed backward JK can counter the Spin, along with a JP starter, but this is not a fool proof method. A far away jump will at times bait a Spin attempt, which is very often blockable since you will land approximately at the same time the Spin will connect at a near full screen attempt, or it will go right under you, so watch for a follow up Spin, or a revert to the Def Jabs.
If you gain significant advantage against this tactic during the match, watch for random cross up throw attempts that are easily countered by an uppercut. This is generally done to throw you off from the Def Jabs and most likely they will revert to it the next round.
Unblockable moves like the Sheeva Ground Stomp, Jax Ground Pound, Cyrax Bombs, or unduckable moves like the Forceball, all hinder the usage of the tactic to some extent due to forced positioning. Depending on the character using the Def Jabs, will dictate the best character to counter with. It's also important to experiment with the entire roster as much as possible. Knowing how to play as a character will help you learn how to play against a character.
Always remember your opponent doesn't necessarily have to be holding Block during Def Jabs and this means they can do special moves during that time. A good test once again, is the sweep, so if they block it standing, they are most likely committing to the tactic, but if they aren't, you will get free sweeps against them unless they are good enough to skillfully Blue Block the sweeps.
Apply these tactics to other characters you may see use this, like Nightwolf, the Female Ninjas, Liu Kang, and Stryker, remember the properties of their special moves, and the punishability as well.
