The Storm of Insurrection
A trip attempt can be made in place of a normal melee attack.
You make a Base Attack + Str + Dex opposed check
Cover and Flanking applies
Size modifier applies (2 x number of categories larger)
For each pair of feet beyond the first two, a creature gets a +2 to opposed trip rolls.
Success: Target falls prone
Great Success: If you succeed by 10 or more, you may make an attack as a free action using the same base attack bonus as the one you tripped with.
Horrible Failure: If you fail by 10 or more, you fall prone