You can enter the space that a foe at least two size categories bigger than you occupy.
Prerequisites: Small or smaller, Tumble 10 ranks.
Benefit: You can move into or through a square occupied by a creature at least two size categories larger than you. You do not provoke attacks of opportunity for doing so.
While you are in a square occupied by a creature at least two size categories larger than you, you gain the benefit of soft cover (+4 bonus to AC) against all attacks, including those of the creature whose space you occupy.
Normal: Without this feat, you can move through squares occupied by a creature at least three size categories larger than you, or a creature three size categories smaller than you (or any creature, if you are Tiny or smaller).
Source: Races of the Wild, page 152.