I hear some bodybuilders saying they see a difference in hardness when they start working on their posing routines.
BUT back in the day in comic books there was a program called Dynamic Tension with a picture of a bodybuilder claiming you could look like that in 10 minutes a day of "dynamic tension" or flexing.Fact is you need the muscle to lengthen and shorten,go through some(can be partial) range of motion with resistance.
i had a nasty broken left wrist/arm years back and was not able to flex my left arm as well as my right. consequently my left arm was an inch smaller than my right arm. i flexed it all the time. while watching tv or doing mindless things. i am now able to flex it just as effectively as my right and can get good contractions in it when working out.
i believe the flexing practice improved my mind muscle connection and now my arms are of equal size.
-JS
i know it def helps in the long run with developing a great mind-muscle connection. And thats one of the hardest things to attain skill wise in bodybuilding. But once you get a great connection your workouts become wayyy more profductive