It all has to do with how the blood behaves.
Try this. Look at your palm with full concentration for a few seconds. It will start to turn a nice pinkish red.
The act of concentration itself (whether on the palm or any part of the body -- internal or external) will cause a quick concentration of blood to that part. This the reason why Qi Gong is good for general health maintenance and even the eleviation of some medical conditions.
Not convinced yet?
Try this. Think of something sexy. What happens? You did not (or rather could not have) will the blood to rush to the genitals to cause an erection. The body merely and spontaneously reacted to your sexy thoughts. The body did not know for sure whether you actually intended to carry out any sexual activity. It just assumes you do and thus prepares itself for it. And after the sex act, it rushes away.
Part of my meditation includes concentration on all the internal organs in a certain sequence and I can actually feel some organs, especially the liver, warm up when my mind is on that particular organ.
I'll rather be happy than right, anytime.