Question: If a little of the 'awrah is uncovered during prayer, does it invalidate the prayer?
Answer: Abu Hanifah says that if less than a quarter of a bodypart (that belongs to the 'awrah), such as 1/4 of the hair of a woman, or 1/4 of the thigh of a man, is uncovered, then it does not invalidate the prayer. If more than a quarter is uncovered, then it invalidates the prayer, unless one covers it immediately.
According to al-Shaafi^iy, uncovering the ^awrah even a little would invalidate the prayer.