Glancing blow

A Glancing blow is an additional hit caused with a two-handed weapon. Glancing blows initially have lower chances of hitting and do significantly less damage than ordinary attacks, but they can hit multiple enemies.

Glancing blows are an inherent property of all two-handed weapons, including quarterstaffs. No Feats or Enhancements are required to enable them, though some make them better.

Basic mechanics
One free glancing blow attack is made automatically against all enemies in a wide arc in front of your character on the first and fourth swing (if you have a fourth swing) of your attack sequence. You must be stationary. It is possible -- in fact, common -- to get both a glancing and a normal attack against your main target. These are rolled separately.

Glancing blow attack rolls use all your normal attack modifiers plus a -5 penalty. Their damage is a varying fraction of your base weapon damage that depends on your Base Attack Bonus:1 This damage has a minimum of 1. Damage Reduction applies against it normally. Glancing blows ordinarily do not cause any bonus damage or special attack or weapon effects.
 * BAB +0 to +10: 20%
 * BAB +11 to +15: 25%
 * BAB +16 and up: 30%

Glancing blow damage, frequency, chance to hit, mobility, and chance of applying special effects can be increased by the Two-Handed Fighting feat line and by some Fighter and Barbarian enhancements.

1 reference: April 2008 forum post