Each person goes to their own limits of strength, flexibility and balance. When we're holding a posture we focus on body sensations. If the sensation happens to be an itch or an ache, then, instead of changing position or putting it out of our mind, we go right into the sensation. In doing so, it can actually resolve. Stop and observe how you're breathing right now.
Yoga emphasizes how to breathe correctly.
We relax both while we're holding the posture and after the posture. Relaxation is crucial for our well being as well as an important tool for pain relief. If you're in pain and you tense up, you hurt more. Instead of automatically grabbing for a pill, in yoga we allow ourselves to feel the pain, and relax into it. In doing so, it starts to dissolve.