What does karate chop in a french bread dough mean?
In context, it means to press down with the blade of your hand, to facilitate folding into thirds:



I put an orange line near the part of the hand you would press with. 

This is not a standard culinary term, by the way&mdash;the recipe author is being cute.

In practice, you could use the handle of a wooden spoon, or press a little groove in with your fingers, as well.