Stronger attacks inflict more damage, but use up more time. Strong attacks will cause the defender to receive a multi-directional arrow 35% of the time. In other words, the defender won't know what direction you pressed for the strong attack.
Medium attacks, are well.. in the middle, this will cause the defender to receive a mutli-direction arrow 25% of the time.
Weaker attacks use up less time, but inflict less damage. However, weak attacks can be put out more quickly, causing more arrows for your opponent to try and fumble through. They also cause the defender to receive a multi-direction arrow 15% of the time.