Underfoot Combat

 
Prerequisite(s): Size Small or smaller, Tumble 10+ ranks
 
Benefit: You can move into or through a square occupied by a creature at least 2 size categories larger than you. You do not provoke an attack of opportunity for doing so. While in the occupied areas, you gain a +4 AC bonus (soft cover) against all attacks, including the creature who’s square you occupy.
 
Source: Races of the Wild