The Middle Way
Being naturally competitive, I'm far more prone to the first mistake than the second. But I've been guilty of both, I'm sure. What's interesting is that both are extremes not just of physical movement, buf of mental state. The first extreme is rigid physically and mentally. The practitioner imposes his will on the practice. And in the second case, the practitioner imposes no will at all; he's just going through the motions.
Finding the middle ground is just as much mental as physical. It's a game of channelling energy without trying to regulate it, of leaving distractions aside without surrendering mindfulness. It's a meditative practice as much as a physical exercise.