The male organ i.e. penile structure is not made up of bones but
contain strong muscles that eat up more blood and retain to stand
up and remain erect while the man is aroused sexually. Said this,
when the man is sexually excited seeing naked woman or during
the foreplay, the muscles of the penile structure retains the blood
and make the organ erected. The best hand exercise to make
penis bigger based on this is stretching it naturally. During the
day, stretch the organ downward (when it is not erected of
course) a few times. This naturally lengthens the structure and
you get bigger organ.
The second answer for how to make your penis bigger
naturally by using your hands is massage it upward when it is
erected. For this, you can manually erect the penis using hands
and once it is stood, you can putt it upward. Do this gently and a
couple of dozen times. You can practice this 3-4 times a day for
3-4 months to see visible results.
The muscles in penis are similar to that of what you have in rest
of the body (say biceps). But then, these muscles grow bigger
when they retain the blood. Building these muscles is quite similar
to that of biceps and triceps. You can use traction and lift it with
erected penis. A very little weight can be tied with glans penis and
as you do a weight lifting using arms, can do with penis.
However, you must take care that you should not use too much
weight there and lift the traction gently.