In punching and stepping, there is a saying to " punch as your foot lands on the ground".
Example, if in orthodox stance, 

Stepping with left foot, you would jab as you land.

In being very picky and accurate, one of my coaches said "I still jab too Late".

1) He said "when your foot lands, your jab should be at nearing full extension."
2) What I was doing, as soon as my foot lands, I being my punch.

Which one of these methods is correct? I'm guessing it's his way, however I'm seeing a variety of ways in Professional matches so wanted t to be sure.