I don't believe that random flexing(clenching) will build gains but it does help when working out, on a side note clenching your abs helps with your posture as well as giving you added energy while doing exercises.
What do you mean by building gains? I am having a very hard time following.
Clenching muscles, that is, flexing muscles during particular movements, helps a lot in terms of performance on certain movements - particularly strength skills like the bench press, pushups, pullups, dips, etc.
Flexing your abs during certain movements does help but it hinders other movements. For example, if you need to arch your back for a walkover then clenching your abs will hurt your performance but if you clench your abs tight just before setting for a front flip then it will help the movement a lot (actually, this is called the hollow position.)
Also I don't believe that the people who visualized tensing their biceps has anything to do with how long you've been working out or how strong your biceps are because as your strength increases so does the 'nerve energy' that's produced, it's all proportional.
It doesn't help people who are experienced because their muscles are already "wired" better than most people...but hey, if you want to think about clenching your muscles in hopes that it will help you then go for it.
