Similar to how I learned the Jab Tap, I learned this technique the hard way - glove to face in the ring.
When I first started boxing, I tended to hold my hands just below eye level. That's not necessarily a bad habit, but my trainer had a hay day with it.
In The Beginning...
Being new to the sport, I didn't have the skill or reflexes to bring my hands up to catch, parry or block what seemed like lightning bolt jabs coming from my trainer. He easily came through my mixed boxing guard - over and over again.
Well, eventually, I got tired of getting hit in the face and reasoned that if I couldn't block his jabs, I'd just hold my hands higher, adopting a much more peek a boo guard at the risk of exposing my body. I have a strong core and figured I could take the punishment the few times that I couldn't get my elbows down to block a body shot.
This actually worked -- for a little while. I held my hands more at the level of my forehead and peeked through the opening between my forearms. My head was completely protected - or so I thought.
Once my trainer wisened up, and it didn't take long, he taught me a valuable tip - again - the hard way.
If you think about my stance, hands held high, body more or less exposed, forearms nearly directly in front of my face, getting a jab in there is pretty difficult. If you were fighting me, you could easily go to my body, but I knew that and I'd be watching for it - I had a plan to adapt to a body assault.
To defeat my guard, my trainer simply said to himself - okay, I won't jab him, I'll pretend to jab and then nail him with a left hook. And that is what he did, over and over and....over.
So every time he threw a fake jab, I braced for impact from the front and somehow he managed to change his jab into a solid left hook that connected every time. I couldn't react fast enough to block the hook when I finally realized the jab wasn't actually going to impact.
Back to the drawing board for me as I was pummeled repeatedly by left hooks the rest of the night.
There was another benefit for my trainer too. He only had to connect once with that left hook to make subsequent punches easier for himself.
Just the fact that he made it through my solid fortress of forearms made me question where his jab was going to actually go. Now, if I made a move to block the hook, he just carried through with the jab. Either way - I was back to getting nailed in the head.
If you perfect this boxing tip - the jab fake - you will have a punch - hook or jab - that will land 90% of the time (or at least until your opponent figures out a way to react to it).
When to use the Jab Fake
This works best when your opponent is in a peek a boo stance. With his arms held high, it is much more difficult for him to react quickly - partly because of the position of his arms, but also because his vision is likely partly obstructed by his own arms and hands. This lends perfectly to faking a punch, especially if you manage to change the punch behind one his blind spots.
If you don't know what I mean by a blind spot, sitting where you are, lift your arms up in front of your face as if you were in a peek a boo stance. Anything that happens behind your arms is partially obstructed from view - those are blind spots.
If you can initiate a fake punch from where your opponent can see to where he or she can't - their instinctual reaction will start and it will be too late to change by the time the punch comes back into view and they realize it isn't where it's supposed to be.
This all happens in a split second so you're not going to consciously see it, but what you might see is a flinch in one direction at which point you go the other way.
Granted, throwing something in a blind spot is more a matter of luck than skill, but it is still something you should strive to achieve.
To summarize, use the jab fake against someone who is squared off in front of you in a fairly upright position, with arms held high partially blocking their view.
Utilizing the Jab Fake
You initiate a jab fake like you would any jab and you have to make it believable (like any sort of feint). Your opponent has to believe a jab is coming at him and it is going to impact. If he believes it and he is in a high peek a book guard, he will brace his forearms for impact (if he doesn't try slipping). If it is not believable, he won't, and you'll have less chance of landing the actual punch.
So - throw your jab, but pull it back about a quarter of the way into it and loop it into a left hook that goes around your opponent's forearm and strikes the intended target. This is hard to do. Stopping the forward momentum is difficult and the more you can loop on the way back, the more powerful the hook will be as you change the direction of the force. Speed is key here.
Another way to aid in making your opponent commit to an incoming jab and brace is to give other cues - such as throwing your shoulder forward. Read the boxing tip on feints for other ideas on making a believable fake.
Practicing the Jab Fake
You can practice this pretty much anywhere. The biggest thing to master is halting the forward momentum of your jab and re-routing that force into a left hook. It is going to feel awkward but with practice you can do it and even get your torso to throw a little extra force behind it. Do this on a heavy bag and strive to achieve a solid left hook after you fake the jab.
Then find a willing partner. Have him spar with you with hands held high. See if you can get him to believe in your fake jab and trick him into receiving your left hook. It will give you a good idea of how much you need to pretend to commit the jab in order to give you the time required to land the hook.
Strike Fast, Strike Hard...Good Luck.