I do it real close to the way its done in the form. I modify a little bit though. I catch the punch with a vertical outward block just ahead of the left foot stepping. As I settle onto my left foot in a modified closed kneel I land the punch just below the floating rib. The settle involves my entire body weight, plus the speed of the punch plus the force of the attacker comming in. All three of these factors will create a devestating blow that will drop an opponent of any size. I have never had any problem making this technique work on any size opponent.
Once again the key is NOT in the step, the speed, the punch it is ALL in the timing and settling or your body weight. If you can learn to develop the "Settle" you can make all the techniques work and hurt.
Remember kenpo originally involved the lower quadrant to generate force! It tends to be left out in todays newer versions kenpo. Its seems to be all about upperbody speed and strength with very little emphasis on legs and weight settle.