Let's not continue to conflate "fajin" only with "one inch punch" which time wise is of short duration. As you see in the photos which show "longer duration" and "short duration" of force. How do we do either or both? If there is a "secret" to success it is to be found by us in our discernment or non-discernment of small differences in sensation that we feel and our subsequent ability or inability to both note and choose options and movement efficiency. In a workshop Master Stephen Hwa said "...the amount of fajing you can deliver is proportional to how compact you can make the delivery..." AND as I have said "... It is an established principle that the smallest difference in sensation we can discern is proportional to the magnitude of the larger sensation..." The "larger sensation" for all of us is the much larger section of the body that is away from the much smaller area that we wish to move.
IMHO: There are no "SECRETS" of Tai Chi, if you want to develop "one inch punch" in a hurryyyy, go home and do slowwww quarter body movement as a punch for 2 hours a day for a hundred days. The thing is there are slim and none people willing to do that...the "secret" is in sheer persistence for a "short duration" of time. Or you can do it spread out over years as a "long duration" with the same persistence.