This skill’s most important effect is to let you cast spells more reliably while being attacked. Technically it helps both skills and spells remain unbroken by attacks, but skills have a relatively low chance of being interrupted in day to day play. This skill doesn’t seem to do very much below 90%.

The in game help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: Con/Siz.
It uses no endurance points.
Discipine is one of the most important traits a 
warrior must possess. With discipline he is able to block
out pain. Even when one is hit incredibly hard, he is 
able to continue whatever maneuver he was executing
before being hit. Hits that make most stop in their
tracks and recollect their thoughts, are simply shrugged
off by those who have this ability.