Underfoot Combat

(Races of the Wild, page 152, DnD 3.5)

[General]

You can enter the space that a foe at least two size categories bigger than you occupies.

Prerequisite

Tumble 10 ranks, Small or smaller

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).