Just remember the Errata out for the feat
p. 7, Knockdown
Insert to end of Benefit:
Use of this feat cannot be combined with Improved Trip to generate an extra attack, and successful use of this feat does not grant an extra attack through the Cleave or Great Cleave feats.
Insert may into “you may make a trip attack as a free action.”
After “whenever you deal 10 or more points of damage to your opponent in melee” insert: with a single attack