I too can feel my brain contract when I focus on it. Once in a while it vibrates too. I also focus on the atmosphere surrounding me and can almost see something in there..
It takes a muscle to move a body part. There are no muscles inside the cranium, as you have admitted. You may THINK your brains are moving around, but they are not. They are just sitting there. Electrical impulses, however, can be felt. Some people who take SSRI drugs, for example, if their dose drops they sometimes feel a brain zap or hum inside their skull. Arteries inside there, too, can be felt when they goof up, in the form of certain types of headaches, for example.
Even though the brains don't move, there is some evidence that a person can control the way their body subconsciously or "autonomically" works just by thinking actively about it. Tibetan monks and other meditation-oriented people can achieve a very high sensitivity to their mind-body connection. In modern medicine, this is known as bio-feedback. You watch a screen that monitors various activities of the body like heart rate. When a person has to deal with a lot of pain, for example, in order to control it without drugs, they'll hook them up, and when the person sees body changes on the screen, they'll actively respond by slowing down their breathing, which alters what they see on the screen, and the forced relaxation helps pain.
In the psychological world, this is sometimes referred to as mind control. "Mind over matter" is a common phrase. Many ordinary people will try to think positive thoughts, in order to affect the outcome of their lives in a good way. I am of the opinion that people have an energy that exists apart from today's measurable methods. The orientals refer to this as the "chi" energy. I have felt it myself, surging up and down my body. I suppose this could be some sort of neurological thing, or I'm feeling the pulse of the heart, or whatever. But whatever it is, it can be felt, and I believe controlled. And I also think it can be projected outwardly in the world... some project it unconsciously to their loved one by a sweet expression, the dog wshiperer on TV Cesar Milan actively projects it to his canines. But could just be that's just a way of looking at things.
So, in summary, your brains don't move. You can't move your brains. Besides, if a muscle was connected to the brain and moved it, what would that accomplish? Nothing. This is because the brain is chiefly an electrical organ that sends out, receives, and interprets sensory signals from all over the body. It has no need to move. On the other hand, the heart does need to move, since it's chiefly a muscular organ that pumps blood around the body.
With the brain, you open your eyes, the "window to the soul," light comes in, and the brain "sees" the picture for you. Some people "die of a broken heart," because the emotional stress will overwork the heart, and it will stop "beating." These phrases hint at the myriad systems at work in a human being's existence, like hormones, electricity, blood chemistry, glands, organs, senses... indeed there is a whole world unto itself inside us.
The human body is so complex, in fact, you cannot discuss one part without discussing the other. This is because, like the world around us, we are interconnected. Just like the river's health affects the land's health, so too within our bodies, nothing is separate, and the health of one organ affects another. A person can get a blood-borne cancer, from mechanisms and realms we still do not understand, and it spreads to the brain and kills it. A polluted river runs down to the ocean, toxins from acid rain or sewage or resulting in "red tide" which is still poorly described, and it can poison the animals that drink the water or need it to live in.
If you have some sort of idea that your brains are wiggling, visit a psychiatrist, they are experts in mental issues, to get at the bottom of this thing that you've noticed about yourself. However, if you're talking about mind control or some of the other things I've endeavored to cover here, then, while I cannot swear as to its scientific value, I hope this will at least steer you in the right direction. If neither is what you wanted, then perhaps a neurologist will opine for you about "brain flexing," in which case you may wish to consult the "Ask A Doctor" section of MedHelp.