Just as training attack keeps your melee attacks from missing, training tumbling attack keeps your melee attacks from being dodged. Since even the smallest of monsters can dodge a lot, this is a very useful skill for improving your damage output. Even 30% in this skill can make a real difference.
The ingame help reads:
Skill duration: random. Type of skill : neutral skill. Affecting stats: De. It uses no endurance points. One of the most difficult aspects of combat, is attempting to hit an opponent who is actively attempting to avoid your attacks. If you are able to calculate your opponents move ahead of time, you may be able to feint and catch him off-guard. This tactic can effectively negate some of defensive maneuvers of your opponent.