Well thats not really true...
In order to hold a bone still if a muscle is contracting, another muscle needs to contract to hold it in place. This leads to a chain of contracting muscles that stabilize the joints in question. This has nothing to do with "energy" but with biomechanics. Clenching during some movements is helpful (like focusing on forced grip in a DL or pullup) but that sort of thing happens a bit more naturally and is more obvious than clenching your muscles for no apparent reason.
Did you even try it? o.O Cause it works, give me your email and I'll send you a copy if you want.
All from Pavel's book Naked Warrior:
"Tension = Force . The tenser your muscles are, the more strength you will
display and build."
"In a recent study, subjects increased their bicep strength 13
p e rcent in three months by simply visualizing tensing their biceps hard but
doing no exercise whatsoever. The only possible explanation for this stre n g t h
gain is greater tension through increased “nerve force ” ."
Interesting:
"A dirty little secret of bodybuilding is that one of the best ways to build the
biceps is with the powerlifting-style wide-grip bench press. Your bis may fight
your tris in doing skull crushers, but they assist your triceps, deltoids, and
pecs in the bench press or the one-arm pushup. In multijoint, high-re s i s t a n c e
e x e rcises, the antagonists often act as synergists, especially in experienced
athletes. In other words, the “brakes” become “engines.”"
"Hit the deck and give me 5
pushups, comrade! Only 5 but of a
challenging variety—for instance,
with your feet up or on one arm .
When you are done with 5, you
should be able to grind out another
couple but no more than that.
Please do pushups on your hands,
not your fists, fins, or fingert i p s .
That way, you will do a better job
of driving the Naked Wa rr i o r
principles of power generation
h o m e .
Note the difficulty of your first set.
Rest briefly. Do another fiver but
with one diff e rence: On the way up,
grip the deck hard with your fingertips. Don’t go up on your fingertips; just
grip the floor so that your fingertips turn white. Do this only on the way up.
Experiment with whether you get the best results by gripping all the way up
or just at the sticking point.
You cannot help noticing that your arms suddenly received a jolt of extra
e n e rg y, as if your tensing fore a rms sent some juice up into your triceps. That
is exactly what has happened. Whenever a muscle contracts, it irradiates a
‘ nerve force’ around it and increases the intensity of the neighboring muscles’
contractions."
"You will increase your strength in any
upper body exertion by strongly gripping
the floor, the bar, etc. What is truly remarkable is that tightening your fists
can enhance your leg strength, as well!"
Sorry, It's really long but worth it.