Answer from Ted Weimann:

Regarding the left jab, some fighters only use it as a speed technique, while others also develop the power jab.  The power jab of course, never develops anywhere near the power of the cross, but you CAN drop opponents with the power jab.  The best way to increase the power of your jab is with timing.  Jab your opponent when he's advancing.   That way you add his body weight to however much of your weight you can get behind the jab.  Not only will this significantly increase the power of your jab, but it will make it appear very fast.  The best timing is to hit your opponent before his advancing foot contacts the ground.  That way more of his body weight will meet your punch.  It will also be harder for him to avoid the punch, and it will be harder for him to safely disperse the power of your punch.

Also, you can increase your power by stepping forward with your left foot.  When you do, strike your opponent before your foot hits the ground.  If you hit him when your foot is contacting the ground, or after it hits the ground, some of the force that could have been in your punch is now going into the ground.  

Another way to increase the power of your jab is to quickly snap your torso forward as you jab.  Both of these last two techniques increase your power by increasing the amount of your body weight moving forward with the jab.  As you know from reading Warrior Speed, as you increase the amount of mass you move forward, you increase your power, but decrease your speed.  

Another method to increase the power of the jab is to incorporate your hips.  This technique is best accomplished without stepping forward.   Begin your jab by quickly snapping your hips clockwise.  This twist is followed by a pulling of the right shoulder back and a pushing of the left shoulder forward.  The straightening of the arm follows to complete the punch.  To an observer, it will appear that all three events occur simultaneously.  

The standard method for increasing the power of your jab is to twist it when you make contact. This twisting will slightly, very slightly, increase the power, the speed (by increasing the reach), and the chance of cutting your opponent.  Keep training and review the book again, and I'm sure you'll continue to get more "sudden improvements".

Ted