Overspanning - Opening the fingers out wide. Here's the theory Normally, your muscles and tendons get blood through capillaries which pass among the muscle fibers. When you tense a muscle, you restrict the blood flow. Once one muscle hurts, all its neighbors tense up, perhaps to relieve the load. This makes sense for your normal sort of injury, but it only makes things worse with repetitive motion.
More tension means less blood flow, and the cycle continues.
Another by-product of the lack of blood flow is tingling and numbness from your nerves. They need blood too. Anyway, when you're typing too much, you're never really giving a change for the blood to get back where it belongs, because your muscles never relax enough to let the blood through. Stress, poor posture, and poor ergonomics, only make things worse. Acute injuries are severely painful and noticeable.